With Gordon Brown at his lowest ebb, begging forgiveness for the failings of his party and the scroungers in Parliament, it will surely reassure the PM to know that he still has the vote of one woman. “Of course I’ll still vote for him,” says the 24-year-old pop megastar Lily Allen, sitting in her dressing room before a secret MySpace gig in Notting Hill, close to David Cameron’s home. (The Conservative leader gave her CD to Barack Obama and told Allen that his own daughter is a huge fan.) “I can’t not vote Labour,” Allen says.

She has said that Gordon Brown is “a nice man” and that she could never become a Tory, although she does say that the Mayor of London, Boris Johnson, is “a cool guy” and that she wants to fulfil her promise, made in the wake of the glut of teenage stabbings last year, to help London’s troubled youth. But not until she has enough time to do it properly. “I could just turn up to something and shake people’s hands and have my photo taken, but I’d rather do something hands-on and actually achieve something.” Then she breaks into squealing giggles. “Boris wrote me a letter recently, but I’m not going to tell you what it’s about.”

So your conversation with him has moved on from the political? “Yes!” And what if Boris stands against Cameron for the leadership? “Oh my Lord! Can you imagine. Could that really be on the cards though? Because David Cameron’s not going anywhere, is he? I dunno, it’d be quite funny to watch the campaign. London’s one thing, but I can’t imagine Middle England voting for Boris, ha-ha.”

Allen has been facing the recession head-on. “I haven’t lost any money, because I’m terrible, I spend, spend, spend, ha-ha. There are loads of Chanel jackets in my wardrobe. I am singlehandedly keeping the economy going.” (She has also bought a stretch of beach in Jamaica, where she plans to build a house.) Still, you wonder if she’s too good for a generation of new Labour-indoctrinated bossyboots when her fans boo her for lighting up onstage. (“No boos!” she replies. “It’s jazz!”).

Fans have free tickets for the most intimate show she’s played in two years. There are only about 300 people — Ed Simons of the Chemical Brothers, her ex, among them — in this small venue, the Tabernacle. Allen tells us that this was where she first went on stage, “in a pantomime, when I was 5. It was Sleeping Beauty and I think I played a frog or a boy. I was always either something shit or a boy.”

Last year a press release was mocked for describing her as “the Wordsworth of the MySpace generation”. If it had called her “the Philip Larkin” it would have been closer to the truth. Allen loves Wordsworth, having memorised him at school (though it’s not clear which one — she went to so many, including the expensive Bedales and Home House, often getting kicked out for being a problem kid who got into drinking and smoking). She eventually gave up on schooling at 15 and spent a wild summer in Ibiza, until the future BBC radio presenter George Lamb befriended her and sent her home to sort herself out — he later became her manager.

But somewhere, throughout those difficult years, there was an extraordinary singing voice trapped in there too. She doesn’t use it much, preferring to talk-sing or rap a bit, but when she lets that tender voice out, the room falls silent. She is always note-perfect. She was signed to a major label when barely out of her teens, but when it tried to hook her up with major-league songwriters Allenhad other plans, uploading demos of her own songs to MySpace. When the label discovered how many people were listening to them, identifying in their millions with her sweet, sarcastic songs about crap sex, mad old grandmas, wanting to look like Kate Moss and “the filth” taking away her driving licence, it realised that it had a star on its hands. And nobody has taken to stardom more naturally than Allen.

Perhaps it was because she had grown up around celebrities — her father is the actor Keith Allen, who spent much of Lily’s childhood in the Groucho Club (his daughter got her own membership when she was 17). Her mother is film producer Alison Owens, who moved the family in with the comedian Harry Enfield after splitting up with Allen’s dad. Or perhaps it was because, when the female pop landscape was awash with bleached blonde Hollyoaks hair and we hadn’t had a decent rebel since Neneh Cherry, here was a young woman who wasn’t afraid to tell it how it is, about sex and drugs and dread. She also wasn’t averse to fights, slagging off everyone from Girls Aloud to Bob Geldof.

But that was then and Allen is now two whole albums older and wiser, a wealthy woman who’s also been through a miscarriage. When we meet I am warned that she cannot speak about the Beckhams or Cheryl Cole for legal reasons — a European magazine has just quoted her as criticising them, something she vehemently denies, and it seems a lawsuit is in process. The day before, Allen played live at a Radio 1 roadshow, where she was told to keep it family-friendly and not sing the rude words in her new single. So she changed a line about spending so much time giving head to kneading bread, which caused her to giggle through the rest of the song.

Allen turned up hours before tonight’s gig to say say hello to fans and sign autographs. She wasn’t contracted to do it and everyone is pleasantly astonished that she bothered. She also requested fancy dress with a London Underground theme, so fans have turned up wearing wings (Angel) or bunny ears (Warren Street). Canada Water, Parson’s Green and Paddington all look as though they took a bit more effort.

Allen herself — dressed as (Queen) Victoria — is too famous to get the Tube any more. “Last time I went on the Underground was New Year’s Eve a year or two ago. I’m a bit haunted by it now because I was trying to get into the centre of town to see my boyfriend and I was on my own. People started shouting and singing Smile at me and I was in tears: ‘Just leave me aloooone’, ha-ha. And people were like, ‘What the f*** are you doing on the Tube?’

“I thought, ‘You know what, I’m not gonna get on the Tube for a while’.”

I wonder out loud if it’s lonely up there at the top, because there’s one song that’s been bothering me — on her recent B-side, Why, she talks about having a phone full of numbers and nobody ever ringing it. “Well, it’s that classic thing,” she muses. “I did sit in my bedroom the other day and think, ‘God, I’m one of those famous clichés: she has everything except friends.’ No, I do have friends. The same friends I’ve always had, in fact. But we all have up days and down days, don’t we? People are probably under the impression that I’m some sort of crazy bouncing-up-and-down teenager that comes in saying really shocking things. I don’t think I’m quite as boisterous as people imagine.”

She can’t even be boisterous on MySpace any more. “I used to like to be able to be quite frank about how I felt about certain situations or people, but then people did take advantage of that and twist things. So it wasn’t direct any more, it was whatever people wanted to make a story out of. So they ruined it.”

“They” are the tabloid newspapers, whose paparazzi wait outside her northwest London flat every day. She does use Twitter though, and used it to create a ticket treasure hunt in cities around North America on a recent tour. She says she wrote all the clues and went out and hid the tickets herself, waiting in person for the fans to find them, and her. Why didn’t she get somebody else to do the dirty work? “Oh, because can you imagine” — puts on the voice of a spoilt starlet — “ ‘I want a rhyming couplet and I want one now!’ Nobody else would have done it for me, ha-ha.”

I tell her that Richard Branson seems to have persuaded Kate Moss to write a tell-all autobiography, which is odd, seeing as the Moss mystique is all about keeping shtum. Whereas the Lily Allen thing is all about being open — so open, in fact, that perhaps she feels no need to write such a book? “I wouldn’t anyway. I think the nature of those books is, you know, you’re f***ing other people over, and I haven’t really got any interest in making money out of other people’s misfortune.”For once, there is no giggle at the end of her sentence.

Lily Allen plays Glastonbury on June 26

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