This is Plan B: How has pop's angry young man suddenly turned into a blue-eyed soul smoothy?

Four years ago, Ben Drew, aka Plan B, was a rapper with an acoustic guitar, a belly full of fire and lyrics to make a navvy blush.

This year, as every year, pop fans could have been forgiven for shrugging their collective shoulders at some of the entries on the BBC's "Sound Of" poll, that increasingly prophetic barometer for the industry's new big things. Topped by Ellie Goulding, aka Dido in Shoreditch clothing, it also made room for two-bit indie types Two Door Cinema Club, tiresome tween-rocker Daisy Dares You, and the toe-curlingly schmaltzy Owl City. Unfair, then, seemed the omission of 26-year-old retro-soul singer Ben Drew, aka Plan B. His rapturously funky debut single "Stay Too Long" hit the top 10 in January, follow-up "She Said" has been A-listed by Radio 1, and album The Defamation of Strickland Banks is one of April's most anticipated releases.

OK, so here's the thing: Plan B isn't actually "new", "Stay Too Long" wasn't actually his debut single, and he had already appeared on the poll four years ago. But really those are just technicalities in the context of one of the most jaw-dropping musical reinventions in recent memory.

Flash back to 2006, and Drew was less intent on charming listeners with honeyed falsetto vocals, and more on grabbing them by the metaphorical throat with cunt- and fuck-strewn lyrical flows. A rapper-raconteur from east London, his bona fide debut Who Needs Actions When You Got Words was a tour-de force of hyper-realistic vignettes about underage sex, crack addiction and stabbing people in the eye with Biros. "My mum was like, 'Can't you talk in metaphors like the Beatles?'" he says, "but I'm not that type of person. I wanted people to hear my stuff and have a learning experience... I don't think there's enough of that now. I'm sure there are people out there, but labels don't want to touch them, 'cos it's not pop enough."

Yet pop is what he has gone, with a wardrobe to match. Where the old Plan B's signature piece was a pulled-up hoodie, now he is all sharp suit and tie combos. For our photoshoot in an east London pub, he swaggers in dressed in a three-piece maroon number with matching brogues and looking every inch the louche, old-school entertainer.

But while Drew may look the part, initially he's not quite as comfortable explaining it. For the first 10 minutes of our interview, he avoids eye contact, choosing to stare at those fetching brogues instead. If he's wary of being questioned, you can hardly blame him. After all, you can hear the cynics cry, what could someone responsible for the line "I show no remorse like a necromaniac/raping a corpse up the anal passage/while contracting genital warts" possibly be doing wading into the safe, commercial waters of Duffy, Adele et al? Isn't this what they call selling out? "There will always be people who doubt you," Drew says, "but if I was to let that affect me, it would be stupid. It would be me making the record for the reasons I made it, and then not releasing it because I'm worried people are going to think things that aren't true. My attitude is: they're not true, so why give a fuck?"

For the record, it was a volte-face not conceived in a big-label HQ, but while gigging his first record, when he would try different styles of songs he had written with his supporting band. For the record, his influences are not Amy and Duffy but Frankie Valli and Smokey Robinson. And for the record, he's play-acting as much as anything else: the new album is a "film for the blind", as he calls it, in which Drew stars as the titular Banks, a soul singer who reaps the rewards of fame and fortune before ending up in prison for a crime he didn't commit. "That way, it made sense that I was changing my style, just for this album and for this character," he says.

There's always been a lot of the chameleon in Ben Drew. At primary school, his first love was jungle music, but in his early teens, he discovered indie rock and taught himself guitar. Next came girls and R&B, prompting him to start writing his own R&B ballads. Having secured a manager aged 16, he then spent four unrewarding years trying to land a record deal as a Justin Timberlake-style smoothie. Finally, at the end of one particular industry showcase, his manager let him play one of the hip-hop tracks he had been working on. "I'd been doing my usual bit and no one was interested, and then I did [this track] as the last song, and suddenly their jaws fell to the floor." And so Plan B was born.

Juxtaposing sensitively strummed guitar with pummelling raps, his sound was nothing if not idiosyncratic. And then there were the lyrics: brutal, mostly character-driven tales of urban decay, yet coursing with a moral indignation far removed from the guns and girls braggadocio he heard in rapping compatriots. Take "Kidz", his first single: inspired by the murder of Damilola Taylor, it found him adopting the voice of a teen killer to offer a pitiful commentary on "the mentality of kidz today". "I was writing songs for kids I felt society had forgotten about – partly through their own fault, because they didn't want to go to school and wanted to get involved in crime, but I saw deeper than that, that their family life was probably pretty fucked up, and nobody had ever shown them encouragement or love... the idea with 'Kidz' was to reel them in with the aggression and swear words and then put it on them in a conscious way."

And what of his own background? Drew says he grew up feeling like a social outcast: deemed a rich kid by his mates on the local estates, because his mother owned her own house, but treated like the "underclass" by the pupils at the suburban Essex school he attended. And though his childhood wasn't "tough in the sense there was no food in the house, it was fucked up in other ways". His dad left when he was six, and he was regularly beaten by his stepfather, giving rise to his own violent impulses. "You have your confidence beaten out of you, you're a little bit shy and then you go to school and someone [picks on you for] that... and then you'll flip out and suddenly you've become your stepdad."

Aged 15, Drew was expelled for throwing a chair at a teacher, and sent to a pupil referral unit for excluded children. A blessing in disguise, there he found the support and encouragement to develop his creative talents. Indeed, as he lost friends to drugs and crime, he credits music with keeping him on the relative straight and narrow. He sold weed for a bit, but only small amounts to support himself while scouting for a record deal, and he says he would have stopped before anything got "too big". "Even if I was unsigned now, in my mind I would still think I was going to make it and I wouldn't do anything to jeopardise that." Writing his first album also functioned as therapy, including as it did a few baldly autobiographical numbers, such as "Mama (Loves A Crackhead)". When I ask him whether he was worried about her reaction to the song, he is definitive in his response. "She put me through a lot, she had a relationship with that guy for a year, and I had to live in that house... I love my mum, but on that particular subject I thought, 'You deserve this.'"

Not that his initiation into the music industry didn't come with its own set of problems. Despite his songs' evident social conscience, they fell prey to misinterpretation. When "Kidz" was playlisted on BBC 1Xtra, listeners rang up in outrage, failing to deduce he was in character. "People actually thought the BBC would play a song saying 'Yeah, it's OK to do these things,'" he says, the disbelief still raw five years on. Elsewhere he had to deal with "idiots on the internet" accusing him of being a "wigga", and he was turned down for a set at Glastonbury because his music was deemed too aggressive. Despite widespread critical acclaim, Who Needs Actions... disappeared from the charts after just one week.

Stung by its reception, Drew's temper flared again and over the next few years, he kept getting into fights and getting arrested. Then, after threatening a man who was "taking the piss" out of him, he was slapped with a suspended sentence. Mortified, he booked in for anger-management counselling. "I realised a lot of my anger was stemming from [the fact] I wanted more respect for what I'd done; I felt like I should be more successful," he says. "Then I looked at myself, and realised I never got into this shit for that, I got into it to express myself. I thought, 'Fuck success, fuck fame.' So when I made this album, I just went out and did what I wanted to do."

The irony, of course, is that the new record could well land him the mainstream recognition he claims to now disregard: quite apart from his richly expressive croon, The Defamation of Strickland Banks contains any number of potential hits, from the finger-snapping "She Said" to the irresistible, Gnarls Barkley-esque groove "Prayin" and string-swept weepie "I Know A Song". In an interview conducted in the wake of his first album, Drew said he wanted his next one to go gold, partly to help him support his family. Does commercial ambition really have no bearing on his new guise? "It's not why I made it [this album], but I feel this music deserves to sell that many, it deserves to be performed to people in arenas, and there's no reason why it can't be. With the first album, there were 100 reasons why it couldn't be," he says.

Undoubtedly, there's been a degree of compromise involved in the making of the new album. Originally, Drew wanted to release a double-album including a hip-hop disc that would tell the same story with "gritty lyrics like before, going into the details of all the nasty shit that would happen in prison". But his new bosses at Atlantic, having recently taken over his label 679, had other ideas. "They said, 'We can't get radio play with that, but we love this soul album, we want to push that.' At first I was a bit insulted, but then I thought, 'This is a major label, this is what's going to make money, the other one isn't, and they're trying to sell my music after four years, and they don't want to confuse people about the type of music Plan B makes.'" As it transpires, he's still squeezed in a few rap breakdowns, while he'll release the hip-hop record proper either as a free download or through his own label Pet Cemetery. And that seems to be the thing about the new Plan B: if not selling out, then savvy enough to play the game when required.

Adding further strings to his bow, he's also been forging a film career as promising as his music one, both as an actor – in Noel Clarke's yoof drama Adulthood and the Michael Caine revenge thriller Harry Brown – and as a director. Following a couple of shorts and music videos, he's just been awarded £100,000 by Film London's Microwave scheme to shoot a hip-hop musical called Ill Manners. You might think the step up to feature films would be daunting for someone of such relative inexperience, but it seems not. "[With my first short film], certain people on set hated my guts, they'd been working in the industry for 50 years, and then this little cunt comes in, and he doesn't even know what a dolly is, but the proof was in the pudding: when the people involved saw [the end result], they admitted it was good. A lot of people treated me like I didn't know what I was doing, but I fucking knew what I was doing." It's an assertion that could sound impossibly arrogant, were it not for the cheeky glint in his eye when he's making it.

Indeed, over the course of our meeting, that initial aloofness has dissipated and a different Drew has emerged: one marked by good humour, fierce passion, and heart-on-sleeve sincerity, as when he talks movingly about holding auditions for Ill Manners at the pupil referral centre he attended. "A lot of the kids weren't suitable, but they loved it. One of the geezers there said, 'Some of them [refuse to] get involved in anything. This is the first thing they've actually been excited by.' For the first time in my life, I felt like it gave me some real purpose."

As he bids me a warm goodbye, I reflect on something Drew said about his changing outlook on life. "I'm trying to be more of a people person. I'm training myself and I've got a lot better. You've got to remember that when I was a teenager, no one gave a fuck about me; people only started caring when I started doing Plan B. Even now I meet people who look you up and down, until they find out you've starred in that film, or written that song, and then they're lovely... but I'm learning to take things less personally." It's this keen awareness of himself as a work in progress that makes Drew so compelling, both as an artist and a person. For if his transformation from plan A to Plan B and from Plan B mark one to Plan B mark two has been dramatic enough, what plans C, D, E, F and G might be waiting in the wings?

Plan B's single 'She Said' is released on 29 March; the album 'The Defamation of Strickland Banks' follows on 12 April. His UK tour begins at the Anson Rooms, Bristol, on 8 April