If you came across Nigel Kennedy in a dance tent at a muddy festival, he would be an instantly recognisable type. From the unwashed hair and outdated slang to the mischievous cackle and tattered clothes, he is exactly like all the old ravers I've ever known – hungover and irreverent, a bit battered. What makes him so thrilling – or maddening, according to taste – is the fact that he is instead one of the most famous classical violinists on the planet. Next week Kennedy will perform at the Last Night of the Proms, where the conductor will be a woman for the first time. But while the classical world has been congratulating itself, its 56-year-old enfant terrible couldn't care less.

"I really don't see what's so important about gender. I think conductors are completely over-rated anyway, because if you love music, why not play it? Why wave around and get off on some ego shit? I don't think the audience give a shit about the conductor. Not unless they've been pumped full of propaganda from classical music writing or something. I mean," he rants on cheerfully, "no one normal understands what the conductor does. No one knows what they do! They just wave their arms out of time."

Isn't he friends with any conductors? Kennedy looks scandalised. "I wouldn't hang with conductors, man. I've got standards!" As his manager murmurs a softly resigned, "Oh dear", it occurs to me that it's a good job people love her ungovernable charge for breaking the rules. Not many minutes later he admits to still taking drugs, and even confesses to – of all things – electoral fraud.

We meet in a BBC rehearsal studio in west London, shortly before the first of his two proms performances. He is rehearsing Vivaldi's Four Seasons, the work that first made him a superstar, but this time he's playing with a group of young string musicians, some as young as 12, many from Palestine. He bounces about, fist-bumping them all – "Killer! Monster! Yeah, man, yeah!" – and they hang on his every word. Kennedy has a bit of a reputation as a diva, but he is incredibly gentle and avuncular with them, and wildly enthusiastic about their talents.

"It's a fantastic group, you know, and they've still got respect for their traditions, so they can play Arabic music, too, you know? How many English orchestra players can play English folk? Fairport Convention have been forgotten a long time ago by the motherfuckers in the orchestras here."

For years Kennedy refused to play the Proms, but he quite approves of this year's programme, which features the first "urban Prom", broadcast on Radio 1, as well as a 6 Music Prom. "Yeah, there's been a fantastic mix of stuff, and I reckon that's what it's all about. The Proms in the first place started out as a stage for modern compositions that were going to be played in front of an audience that wouldn't normally go to classical music." He's a little worried about the last night's traditional flag-waving, though. "This jingoistic thing, yeah, I'm not completely into it, you know. This thing about Rule Britannia, I'm hoping that most people don't really feel like that and are just having a laugh." The only jingoism of which Kennedy approves is the football variety. He'd been asked to play You'll Never Walk Alone for the last night – "But I'm not doing that. That's a Liverpool song, and I'm a Villa fan. I'm not going to play that shit!"

He is wearing a dilapidated Villa shirt over the top of a County Kerry football shirt, above baggy grey tracksuit bottoms that may well not have seen detergent since his hair last saw shampoo, which he reckons must be more than 30 years ago. He's been cutting his own hair for that long as well – "'Cos it's cheap, and that way I don't have to listen to some stupid small talk" – and his slang dates back to an even earlier era. "Monster. Top shit. Killer. That's it," he grins. He can't see any point in updating his repertoire, and thinks the old jazz club lingo will be back in fashion again before long anyway. "Yeah, man. Yeah."

The other musicians in his rehearsal were handpicked from Kennedy's own orchestra, the Orchestra of Life, which he founded in Poland nearly 10 years ago when he and his second wife, who is Polish, went to live for a while in Krakow. He loved the Poles' gutsy old Eastern-bloc spirit, but then "the globalisation sort of thing" put him off and they moved back to the UK. "The spirit and the character that used to be there in abundance just went. It's just like the same changes which Thatcher managed to introduce in Britain in the 80s. I've seen them happen in Poland in the last few years. They love Thatcher over there. So you've got a very unpleasant, full-of-themselves type whose fathers have sold their land and they've got a Mercedes and they think they're everything. All that typical posing bullshit."

Even here in Britain, he's disappointed with the way politics have changed. He still supports Labour, "But it's not really a Labour party any more, is it? So there's nothing left to vote for. If Tony Benn was there, that would be someone worth it – or that [Jeremy] Corbyn guy. You know, there's one or two left who's like decent ones." Kennedy lives in Glenda Jackson's north London constituency, where she only won by 50 votes at the last election, so I ask if he voted. "Oh yeah." Then he adds, quite casually, "In fact, my wife wasn't there, so I got another friend to go and vote for Jackson with my wife's voting card." Seriously? "Yeah, yeah, man, and it was really worth it in that case."

Politics aside, he plainly adores the young Poles in his orchestra, and after performing at the Royal Albert Hall they were all going to be invited back to his house, along with Kennedy's guests in the audience, for "a proper jam, yeah". All of them? "Yeah, I do that after every gig in London. My house is going to be pumping, I tell you." How late will it go on? "Probably 'til about eight in the morning, yeah." The last big post-concert jam at Kennedy's house involved three Balkan bands and their relatives, "so there was like 50 people all singing and playing at the same time and the walls are still vibrating from that. Those Balkan melodies are just like from father to daughter to son, it's just amazing, passionate melodies, amazing stuff."

He has tried to stop partying, "every now and then", but it only ever lasts a week – and this, he admits, hadn't been one of them. "Nah, these last couple of nights have left me a little bit fuzzy," he offers ruefully. What was he doing last night? "As much as possible," he laughs, with a knowing look. Could he be more specific? "I don't need to man, don't need to." He is winking at me like mad. "Every area, man, yeah. Got to be done, someone's got to do it."

I thought he'd given up class A drugs years ago, so ask if he takes anything stronger than cannabis these days. He hesitates. "Well, not on a regular level. But I think you should never say no to anything, because that's just, like, completely limitative." I wonder what he makes of people who say that middle-aged hedonism must denote some sort of emotional void. "They're boring fuckers, aren't they?" he hoots with laughter. "You know, everybody's going to die at some point, but I seem to be in as good a health as anybody else of my age."

The people whose psychological health Kennedy does worry about are his contemporaries from the elite music schools he attended. "Once they reach their 50s, or even in their 40s, they reach this kind of plateau and then quite a lot of them end up even at that point having nervous breakdowns and stuff. And I think it's because they've got such a narrow agenda that playing becomes a repetitive process instead of being a voyage of discovery. If you're discovering new things then you've got something to look forward to all the time."

He adopted this philosophy at a precociously early age. Kennedy was only six when his mother, a music teacher, auditioned him for the Yehudi Menuhin school, where he spent the next nine years as a gifted troublemaker. He then studied for three years at the Juilliard school in New York, but would bunk off to hang out in jazz clubs, where he seems to have ditched his middle-class vowels in favour of the geezerish twang we know today. When the legendary jazz violinist Stéphane Grappelli invited the 16-year-old to perform with him at Carnegie Hall, his teacher warned that it would cost him an imminent recording deal, but after downing a bottle of whiskey Kennedy went ahead and played. "And my teacher was correct. Some guys from CBS decided after hearing me play that shit that I wasn't the right type of cat to do the classical thing they were thinking of me for." Did he mind? "Not at all. Because I'd kind of made up my mind I was going to do it my own way."

So he took up busking. "Well, if I came over to the BBC for a recording I'd get paid £50. I could earn that in two hours on Fifth Avenue. And it taught me a real lesson, busking did. That if people like the music you play, they'll listen to it – and they don't give a shit whether it's called classical, jazz or anything. That there's an audience who will listen to music if they like it, but will walk past if they don't. And that's a good metaphor for what the real life is with music – communication with larger numbers of people, rather than just the classical aficionados."

He was reasonably well known within musical circles throughout his 20s, but in 1989 he released an album that changed his life for ever. He had noticed that Vivaldi's Four Seasons shared the structure of most pop records – 12 tracks of roughly three minutes each – and released a recording that featured Hendrix-esque sound effects, sold 2m copies, entered the Guinness Book of World Records as the bestselling classical work of all time, and turned him into a global household name.

He admits that the endless mockery of his accent, and accusations of punk affectation, used to annoy him, but says it doesn't any more. "All you can be is just be yourself," he shrugs, "and if people think you're putting something on then so be it." And as he points out, "A lot more people listen to classical music now than when I started out," which he puts down in part to the appeal of his persona. And what about the prima donna reputation? "The only time I'm fucking difficult," he grins, "is when people are telling me what to do."

As long as you don't try to make that mistake, he's terrific company. One thing that doesn't seem to fit is a story from a few years ago about him rebranding himself as simply "Kennedy" – which seems oddly grand for a man wearing what are essentially rags. "It was a complete farce, all of that," he hoots with laughter. "It was a fuck-up." He'd discovered that the novelist Iain Banks wrote science fiction under the name Iain M Banks, and thought he could be Kennedy when playing classical music, and Nigel Kennedy for his more experimental hybrid works.

"Yeah, but it never worked and everyone got the names wrong. Then people thought, 'Why doesn't he want the name Nigel?' Then it all just got stupid, so I gave it up." He lets out another great cackle. "So yeah, just let them call me what the fuck they like."

Nigel Kennedy's autumn tour starts in Lowestoft on 12 November and ends in Truro on 20 November. Tickets: eventim.co.uk