'Look," says Frank Turner, struggling to maintain his cheery demeanour after 20 minutes of questions. "I'm a classical liberal. I like John Stuart Mill and Adam Smith. And most of all, Benjamin Franklin and Tom Jefferson. That is my politics."

Having spent eight years building a cult following on the back of his earnest folk-punk rebel songs (covering such topics as atheism, drinking himself half to death and the emancipating power of rock'n'roll), the 31-year-old from Hampshire feels under attack. For a start, there are the punk purists angry at him for playing Wembley Arena last year, and for charting with the album England Keep My Bones in 2011. "The very first time somebody called me a sellout," he shrugs, "was when I did a tour in a van that had seats in it." But mostly, Turner feels targeted by the media – specifically the Guardian. Or rather, the torch mob we accidentally rallied against him.

In a blog last September, a Guardian music writer gathered together quotes from interviews Turner had given, from 2009 to 2011. In these, Turner declared: "I consider myself to be pretty rightwing"; "The BNP are a hard left party"; "I think socialism's retarded"; and "Leftist politics lead to the misery of many, the crushing of the little guy". Immediately, Turner was swamped with abuse. "I went through a two-week period where I was getting close to 100 death threats and hatemail a day," he says. "Really vicious, horrible shit, all because of expressing an opinion contrary to the mainstream of thought within the entertainment or creative world. It was fucking horrible. I don't want to go through anything like that again. My skin has thickened. I've got much better at weathering internet storms – and they pass very quickly. The internet is, generally speaking, a fantastic thing, a wonderful boon for humanity. It gives everyone a voice, which is a double-edged sword. The instinctive egalitarian democrat in me thinks that's a good thing, but at the same time it means that every bored, ignorant, bile-filled arsehole can send me hatemail. And they do."

The day after the Guardian blog, Turner took to his website to clarify his position. "Incidentally," he wrote, "here's some things I'm not: 'Tory', 'conservative', or 'Republican'." He said his politics were "based on principles like democracy, individuality, equality of opportunity, distrust of power and, above all else, freedom, including economic freedom". He had shifted, he said, from being an anarchist in his 20s to now being a libertarian, and he called the BNP "repugnant … they are a socially rightwing/racist party, but their economic policies are pretty authoritarian leftwing. I happen to oppose them strongly for both of those reasons."

We're sitting in a restaurant in London's West End. Holding a lager between fingers tattooed "Freeborn", in tribute to Sunderland's 17th-century "leveller" Freeborn John Lilburne, Turner certainly doesn't look like a Tory. Indeed, despite his oft-mentioned Eton education (on a scholarship), he recently told NME: "David Cameron is a twat … I wouldn't vote for that cunt."

I ask if he stands by his 2006 song Thatcher Fucked the Kids, in which he blames Maggie for creating generations of thuggish, thieving teenagers and uncaring communities. He winces. "I regret writing the song simply because I started getting a lot of people coming to my shows who didn't give two shits about my music. I'd just said something they agreed with. There's a fair amount of analysis in that song that I would 100% stand by, but writing that was almost dipping a toe into the world of being a protest singer. I tried it, I really didn't like it, and so I did something else with my life."

Indeed: three years later, you were calling yourself rightwing. A sigh. "If there was one statement I could retract in my entire life, it's the use of the word 'rightwing'. That word has been stigmatised to mean 'bad guy' and I wish I hadn't said it. To a degree, I was attempting to be provocative when I said it and fuck me did I succeed – because now everyone hates me for it! I'm just not particularly leftwing."

He has said he regards state funding of the arts as a greater evil than commercial sponsorship. How does he equate that with playing the publically funded Olympics opening ceremony? "Fair question. I didn't get paid anything to do the Olympics. In fact, I lost quite a lot of money, because it's a very expensive thing to be a part of. But I got asked by Danny Boyle and I thought it was a good idea. I do think that, if you're going to cut benefits, I'd rather cut benefits to people putting on plays in Islington than to people who live on housing estates. I think we should spend our money helping the poor and the sick and the needy and the unemployed. I've never had any state funding for any of the art I've made and I'd never ask for any. It's a double standard for people to be virulently against Coca-Cola funding something and not be against the government funding something. They're both large conglomerations of authority. I don't like large conglomerations of authority."

Passions run high; concessions are acceded. We agree that the leftwing ideology that Frank imagines "crushing the little guy" is essentially Marxism. "I have friends who call themselves communists. I find that difficult personally because I think communism is an ideology with an awful lot of blood on its hands. The non-Marxist British left is a fantastic tradition: it's all about non-conformism and voluntarism. The advances of the unions are great advances in human society."

Turner clearly has his passions, a fact underlined by his fifth and best solo album, Tape Deck Heart, released this week. Recorded with Muse producer Rich Costey in Burbank, LA, it's a brutally open breakup record inspired by Arab Strap, the National and what Turner calls Bruce Springsteen's "brave and excoriating" 1987 divorce album Tunnel of Love. "I had me some unhappy times in my personal life," Turner says. "It's that classic thing: as everything goes well in my career, behind the scenes other things have to give. But it's not about being the victim, it's a record about being the perpetrator. I'm not saying I'm a bastard and I fucked someone over, but I certainly say I screwed something up. It's this great secret that everybody in bands tries to hush up: never date musicians. Jesus Christ, avoid us like the plague. We're awful shitheads."

Costey drove Turner to extremes. "He made me do 42 takes of one song. I usually do four, or five if I'm feeling Diana Ross. I wanted to kill him most of that time." The result – even for someone who's built an arena-shaped fanbase by treating his songs like a drunk emo's diary, splaying out his every flaw – is a riveting piece of raw-hearted punk pop. "Give me one fine day of plain sailing weather/ And I can fuck up anything," he yells on Plain Sailing Weather. Elsewhere, he sings of relapses into the drug "bad patch" that inspired his debut solo album, Sleep Is for the Week, in 2007. "I'm pleased to say drugs are not part of my life any more, but they certainly made a comeback at various points in the last couple of years."

He also tackles the spiritual desolation of life on the road ("a soulless way to live") and a clinging ex ("the person that has the presidential hotline to fucking you up"). Then there's his teenage self-harming. "Like a lot of people, I feel slightly embarrassed by it – because it's such a concession of weakness. I don't have an enormously high opinion of myself. It's a constant battle not to get too lost in self-criticism, self-loathing."

He drains his lager. "Charity or altruism is not something I like to brag about," he says. "That's how my mother raised me. But I spend my days trying to help people out. Yesterday was my day off and I spent all day sending emails. I organised a few benefit shows. I had some beer shipped to a guy's funeral. I helped out a guy whose son was killed in the Sandy Hook massacre with some tickets to a show. I put together a thing trying to help out a school that does music for kids who can't afford lessons.

"Having spent seven years approaching my career like that, to be suddenly called a massive sellout because of a selective bit of quoting was fucking upsetting. I believe in judging people by their actions and I do my best to be an egalitarian who tries to help people. That to me is infinitely more important than any opinion I might have about politics – and anything I might have said in an interview."