Welcome to my country: Can America's sweetheart Carrie Underwood win over the UK?

Carrie Underwood is a vegan, Christian, reality-TV singing superstar. She reveals what's behind the squeaky-clean exterior to Guy Adams.

Small, perfectly formed, and with a voice like warm treacle, Carrie Underwood saunters into her manager's hotel room with a "Howdy!" and a toss of her hair. Then she sits down, and poses winsomely for our photographer. Above the sofa hang two sepia photographs of a female nude, which according to her publicist are "not very Carrie", but which unfortunately happen to be bolted very firmly to the wall. We are therefore asked, nicely, to ensure that pictures showing her adjacent to a female nipple do not make it into print.

These things matter in the sometimes surreal, sometimes splendid world of Carrie Underwood, a gorgeous, God-fearing blonde from rural Oklahoma who is stupendously famous in her native land and almost equally unknown to the rest of the planet. Top of the charts, on the far side of the Pond, for the best part of a decade, her squeaky-clean persona is an embodiment of an apple-pie brand: the small-town farmer's daughter who owes her fame to an extraordinary set of lungs and a priceless knack for crafting catchy country songs about faith, family, and the all-round greatness of America.

We meet in late May, shortly after her new album, Blown Away, has knocked Adele off the top of the US charts. Our rendezvous takes place at the Sunset Marquis in Hollywood, a local landmark which is also "not very Carrie", since it boasts a loud, proud history of celebrity debauchery. Courtney Love wrote a famous suicide note to Kurt Cobain there; Dave Gahan of Depeche Mode overdosed in an upstairs suite. Everyone from U2 to Eminem has got plastered at its whisky bar. Meanwhile, Underwood, a resident whenever she's in LA, is fond of its landscaped flower gardens and discreet underground parking facilities.

"I get the impression," I tell her, "that you don't really embrace the rock'n'roll lifestyle." She giggles, obligingly. "I know!" comes the reply, in a warm, Southern accent. "Isn't it pathetic? Even in high school, I was an angel. Never went to parties. Never snuck out. Didn't drink alcohol. None of that." She takes a sip of mineral water. I notice her enormous diamond engagement ring, equally sparkling wedding ring, and immaculate American teeth.

Despite, or perhaps because of, her wholesome credentials, Underwood, who is 29, now ploughs a lucrative furrow. In the US, she's everywhere: on billboards, magazine covers and every pop station you tune an FM dial to. Four days before our meeting, she did Oprah, talking freely about married life with her square-jawed husband, Mike. The next day, she sang at the Billboard Music Awards in Las Vegas. Tonight, she'll be in the studio audience for the American Idol final, before heading to Nashville to headline a country music festival. In a busy working year, she'll stage over 100 live performances, filling 15,000-seat stadiums with moms, pops, and their enthusiastic kids. Forbes magazine puts her income at more than $20m (£12.9m) a year.

For all her wealth and celebrity, Underwood turns out to be disarmingly sweet: charming, enthusiastic, and without even the tiniest hint of entitlement. She travels with an entourage of just six – roughly one-third of the quota of your standard pop diva – drives a three-year-old Ford Explorer, and still insists on doing her own cooking and laundry because: "I don't like the thought of someone else rifling through my clothes". In our hour together, she's also sharp, inquisitive and full of questions about London. She is due to arrive there this week, in preparation for 21 June, when the Royal Albert Hall will host her first-ever UK concert.

The trip will last almost a fortnight, and marks the start of a carefully co-ordinated attempt to sell Carrie Underwood to the British public – a first step, presumably, towards world domination. It also marks an interesting cultural experiment. Despite her stratospheric success back home, where, since 2005, she has sold around 15 million records, made three number-one albums (plus one that reached number two) and earnt five Grammys, Brand Carrie has so far remained a purely American phenomenon. In fact, her new album, Blown Away, is her first record to even get a UK release (it's out here on 18 June, on Sony Records). So, I ask, why did we have to wait so long?

"To be honest, country music isn't really... um... it fares much better in the US than it does in other countries," she tells me. "I have always wanted to take country music to other places, and to perform in other places. But I felt it was most important to make sure that I felt comfortable at home, and had established a career here first. Now that I have, I think, done that, I'm ready to go overseas. I'd like to consider myself someone who can potentially reach an audience outside of normal country music fans."

Form suggests that may be a challenge. Steeped in star-spangled tradition, 'country' has traditionally proved a tricky sell outside of the genre's core demographic: wide-arsed, Bible-thumping inhabitants of the American Midwest. There have been plenty of exceptions, of course. Johnny Cash made it big in the UK (and everywhere else) in the Sixties; Shania Twain managed a similar trick in the Nineties. Today, Brad Paisley is more than capable of filling an arena in any major British city. But for every cowboy-booted star who successfully exports, dozens never trouble the charts anywhere except the US.

A cynic could poke holes in Underwood's overseas potential. They could argue, for example, that her 'signature' hit, an unashamedly anthemic track about high-school romance called "All American Girl", won't go down quite so splendidly in rainy London as it does in, say, Texas. They could point out, probably rightly, that the evangelical Christians who enthusiastically propelled her second-most-famous track, "Jesus Take the Wheel", to the top of the country charts will be somewhat thinner on the ground in secular Britain than they are in the Deep South. They could say all that, but I think they'd still be wrong.

For one thing, Underwood's oeuvre stands up to critical scrutiny. Her new album, most of which she co-wrote, may have a whiff of cheese about it. But it also boasts some surprisingly layered, often quite beautiful, pieces of music. The title song, "Blown Away", tells a twisted story about a girl from an abusive home in Oklahoma, who locks herself in the cellar as a hurricane approaches her hometown. In a dark, strangely uplifting narrative, it ponders the protagonist's decision to leave her drunken father – who appears to have either murdered the girl's mother, or driven her to suicide – asleep on the couch.

Naysayers would also be ignoring Underwood's ability to, in the vernacular of record executives, garner 'crossover appeal'. She achieved fame in 2005 by winning American Idol, the TV talent show which for most of the past decade has been a national obsession, and the most-watched thing on US television. An audience of roughly 30 million saw her triumph in the final, witnessing her transformation, during the programme's run, from poodle-haired college student, who'd learnt to sing in a Baptist church, to an assured performer Simon Cowell described as "potentially, the best we've ever seen" in the history of the show, on any continent.

Cowell put Underwood's success down to her stellar vocal range, remarkable stage presence, and "likeable" personality (she jollified her first audition by revealing that she'd been born with a third nipple). She underlined her possession of the common touch by winning the final of the contest by a record margin. Then, after nipping back to college, to finish off her degree, she recorded a debut country album, and went on a nationwide tour. Since then, she managed to sell more records than any other Idol winner in the show's 12-year history.

"Carrie's unique," says her agent, Jeff Frasco. "She has an amazing voice, an amazing talent. After she won, she purposefully decided that she wanted to be a country artist. That is what she loves, and what she is. So we made sure she became credible in that genre and immersed herself in it. But she also has worked hard to reach out beyond the typical fan-base. She's very good at that. She wants to have a 20- or 30-year career, and she totally understands how to make that happen." Thanks to the internet, she already has pulling power in the UK: when tickets to her Albert Hall gig went on sale, they sold out in 90 minutes.

I ask Underwood what she thinks has kept her on an upward trajectory, where other TV talent show winners have faltered. "First of all, I've been very lucky," she says. "Secondly, the best people in the music business really do work with the TV show, so I have got to work with them." She has always been part of British impresario Simon Fuller's empire. "It means I've had very talented people round me from the start. Maybe some of the other winners who've come off Idol have, kind of, deviated from that a little bit, and tried to do their own thing. But I've always loved the show and still love everything about it."

Plenty of people share her enthusiasm for Idol; but not everyone. In 2008, a woman called Paula Goodspeed, who had been publicly humiliated in an audition by Cowell a few years earlier, committed suicide in a car parked near to judge Paula Abdul's home in Los Angeles. The incident sparked a public debate over the show's unforgiving treatment of the deluded, eccentric, sometimes vulnerable contestants (Goodspeed reportedly had mental health problems) upon whose dreams it tramples. Does Underwood ever find it exploitative?

"Well." She pauses. "Look... You know what you're getting into when you go on the show. It can really work out. Or they can make you look stupid. And to be honest, they've actually veered away from that. In the past couple of years the auditions have been a lot more about the talented people [than the failures]." Idol can still be difficult to watch, she admits. "I'm sure lots of people still watch the early rounds to see the bad auditions. Sometimes, I do wonder why these people don't have friends, or family members who will tell them: 'Maybe this isn't the best idea. Maybe you should stay home rather than going to try out for the show. Let's go get ice- cream instead'."

Born in 1983, Underwood grew up the youngest of three sisters. Her mother, Carole, was a school teacher. Her father, Stephen, worked in a paper mill. They lived on a smallholding in Checotah, a small town in a conservative, rural part of Oklahoma, which is in turn one of America's most conservative states. The family worked hard, played hard, went to church on Sundays and kept animals to help pay the mortgage. "We had a few cows, and they'd have calves, and we would sell the calves," Underwood says, when I ask what she remembers most from childhood. "I loved those cows. They were like big dogs to me. I bottle-fed them. I gave them names. I could pet them."

Since being a teenager, Underwood has therefore refused to eat meat. A year ago, she upped the ante and became America's most famous vegan. A portion of her personal fortune is now devoted to an animal centre she built in Checota, though she describes herself as a relaxed, rather than tub-thumping, animal-rights activist. "Often, older people from my home town will come up and say 'What are you doing? You don't eat meat!'," she says. "They're almost wagging their fingers at me, like they think it's wrong for an Oklahoma girl who sings country music not to eat meat. I hate it when someone does that to me, so I try not to force my opinions on other people, either."

Life in a 24/7 celebrity bubble has other costs. Underwood sees her friends and family less than she'd prefer. And like any public figure, in the era of camera phones, she finds it hard to slip below the radar. "I love it when a fan will just say, 'Hi!'. What I don't like is when they'll secretly follow you around, taking pictures, slyly sneaking around the corner." Shortly after she won Idol, an acquaintance sold personal pictures to a supermarket magazine. In the ensuing years, her love life became a tabloid obsession. "Every few weeks, I'd read about 'my steamy new romance', often with people I'd never even met."

Underwood's relationship status is no longer tabloid fodder. In 2008, she was introduced to Mike Fisher, a Canadian professional ice-hockey player who was friendly with her bass player. They swapped numbers, spent three months getting to know each other via telephone (she was on tour) and had their first 'date' on New Year's Eve 2008. By the end of 2009 they were engaged, in 2010 they married, and ever since they have divided their time between Nashville, where Fisher's home team plays, and a cosy cottage in rural Tennessee full of open fires, scented candles, and framed pictures of their favourite Bible verses.

The Fisher household contains two lapdogs, Ace and Pepper (they come on tour, but "don't do red carpets because they are animals not accessories"). She plans to have children in the next few years, and with Mike, has bought 400 acres of countryside near their cottage, where they plan to build what tabloids might call a 'dream home' of epic proportions. Landscaping is already underway. The other day, in a sequence set up for the cameras, Oprah Winfrey helped plant a tree in a spot they hope to one day use for family picnics.

It's lunchtime, now, and a knock on the door informs us that Carrie Underwood must leave in five minutes, to prepare for that evening's trip down the red carpet. Before she goes, I ask about a topic she calls the most important thing in her life, and one of the recurring themes in the music she now hopes to export to our shores: her faith. She grew up Baptist, she tells me, saying it offered an old-fashioned, didactic take on Christianity, but these days attends a liberal, inter-denominational church which "embraces all sorts". She and Mike pray every morning, and before every meal. She also prays before every show, often with bandmates.

Does she ever struggle, I ask, to square her religion with the progressive, gay-friendly mores of the music industry? "Our church is gay-friendly," she responds, seeming suddenly energised. "Above all, God wanted us to love others. It's not about setting rules, or [saying] 'everyone has to be like me'. No. We're all different. That's what makes us special. We have to love each other and get on with each other." Then she offers a totally unexpected, unequivocal endorsement of gay marriage. "As a married person myself, I don't know what it's like to be told I can't marry somebody I love, and want to marry. I can't imagine how that must feel. I definitely think we should all have the right to love, and love publicly, the people that we want to love."

With that, Carrie Underwood gets up to leave. She's been sweet, and helpful, and far nicer that I had any right to expect, so I maybe shouldn't have been be all that surprised to hear conservative America's favourite pop star stand up, loud and proud, for the rights of homosexuals. At the Albert Hall, they'll no doubt love her for it. But I can't help but wonder how it'll go down back home in her native Oklahoma.