When Matthew Collins heard what was happening in Norway in the late afternoon of 22 July, he headed to the offices of his employer, Searchlight, the anti-fascist magazine. There, he and other staff sat up all night watching Twitter and the internet, talking quietly to their informants in Scandinavia.

It took time for a portrait of 32-year-old Anders Behring Breivik to emerge, but it was a picture Collins recognised clearly. For six years, Collins was himself at the heart of far-right groups in the UK: the south London organiser of the National Front (NF), a close confidant of the leadership and seen by some as a potential future leader of the party. He was also a volunteer at the British National Party's (BNP) head office and active on the streets with the violent neo-Nazi street fighters Combat 18, whose numbers one and eight stood for "A" and "H" after Adolf Hitler.

"Breivik's concerns were on a different scale to mine," Collins says. "His manifesto is obsessed with global issues. Mine were much closer to home, more about the end of my street than the world. But essentially they had the same aim – acts of violence that will lead to a civil race war."

Collins' remarkable journey to committed anti-fascist campaigner from a Mein Kampf-reading racist – not just a white working-class boy who shaved his head and flirted with far-right politics, but an active and violent Nazi – is told in his new book, Hate: My Life in the British Far Right. Introduced by Billy Bragg and endorsed by Dagenham MP Jon Cruddas, it is a timely riff on the past 25 years of the UK far right.

As well as being a tale of very British Nazism – a story of smoky pubs, drunken punch-ups, mindless hooliganism and paranoid conspiracy – it also offers a powerful insight into the Norway massacre.

For Collins, 39, even the emerging portrait of Breivik – who writes in his manifesto about "the absence of fatherhood" – sounds a deep echo. "Breivik's father left when he was one," he says. "It's a common thread in extreme-right terrorists, looking for family and security. It usually involves blaming modern society for the breakdown of the family. Timothy McVeigh, the Oklahoma bomber, and David Copeland, the Soho nailbomber, fit the same pattern. There is a shared sense of rejection."

Collins' own father left the family – with his teenage babysitter – when he was five years old. He looks up from his drink. "So, yes, I was one of those too," he says.

The National Front had the tough, violent, but often fatherly male figures Collins was looking for. "For me, there was always this search for male role models," he says. "We had excellent female role models who did all the cooking, cleaning, carpet fitting. But no men to look up to on our estate.

"The far right gave me a safe environment. There was lots of violence, but they elevated me. They took me into pubs at 15. I would be picked up from home in a car, taken to clubs. They taught me things. They treated me as if I was bright."

He shrugs. "The Labour Party never came knocking on our doors."

By 1987, at the age of 15, he was going door to door on mixed-race housing estates with a copy of the National Front newspaper The Flag in one hand "and a wheel brace in a plastic bag in the other". His favourite slogan was: "If you want a nigger neighbour, vote Labour." He shakes his head. "I'd only read one book – Mein Kampf – but that was more than most of them."

Most of all, being a Nazi gave Collins, one of Thatcher's children festering on a 1980s council estate without prospect of work or betterment, a feeling of intense power. "You would strike fear into people," he says. "You could feel how you physically intimidated someone."

His bitter hatred did not just extend to black and Asian people. The aim of a night's drinking would be to "smash glasses into the faces of anyone even remotely progressive, or with glasses or, worse still, a ponytail".

His bloody damascene moment is overwhelmed by shame. It came at a 1930s library building one summer evening in 1989, when the BNP disrupted a public meeting in Welling, Kent. In British fascist folklore, that night became known as the Battle of Welling and its "soldiers" were celebrated as heroes.

The truth, Collins says, is that a cowardly group of 40 or so fascists armed with hammers smashed up a room full of mainly women.

In his book he recalls "one man after the other laying into a small group of women, hitting them with chairs and hurling tables at them .... It was a bloody massacre. People were lying on the floor helpless, being stamped on, kicked and hit with objects picked off the walls and floor. A pregnant woman was locked in the toilet and the BNP were trying to kick their way in to get at her and her unborn baby."

Collins, a veteran of the far right at just 18, had taken part in violent attacks before, fighting men in pubs and on the football terraces. But the scene at Welling Library deeply challenged the nobility he associated with the far right's cause. "I couldn't see what freedom of speech and fighting for British democracy had to do with stamping on little old ladies' heads," he says. "It was real hatred. I began to see it was all about destroying people's lives. Violence was the only way they could affect change.

"I was standing in the library watching people getting their heads kicked in for attending a debate and discussion. I thought: I'm on the wrong side."

After Collins was hailed by his NF and BNP colleagues as a hero for the attack at Welling, he found himself in a phonebox calling Searchlight. One by one, he told the magazine the names of the people who had carried out the violence. A polite Geordie voice thanked him. "Everything sort of changed at the moment," he says. "But my intention wasn't to be a spy."

Over the next few months he rang Searchlight a handful of times. Some days, he became so confused he had to steady himself in front of the mirror with the words: "I am still a National Socialist, I am still a white Aryan, part of the master race."

Before long, Collins was passing secrets face-to-face that would lock away violent fascists and help cause the early 1990s far right to implode. Searchlight paid him in book tokens so he could educate himself.

His role as a mole led to direct threats to his life and a trip to Australia, hastily arranged by Special Branch, where he lived in hiding from 1993 to 2003. He came back to take part in Dead Man Walking, a BBC documentary that brought him back into the open.

In the eight years since his return he has worked full-time for Searchlight, where the deep friendships forged during his years as an informant are still very much in evidence. During that time, he has seen UK fascism turn full circle. By the late 1990s, the street fighters had begun to seek power through council chambers under the guise of outwardly democratic parties like the BNP. But with the collapse of the BNP following crushing defeat in the 2010 council elections, fascists are once more on the streets.

Searchlight has been voicing concerns for months about the English Defence League (EDL), the far-right street-protest movement Breivik enthused about in his manifesto. "This Government is not taking the EDL seriously," Collins says. "These are working-class people who have run out of hope and understanding. If it wasn't radical Islam it would be something else. Increasingly, they have been overrun by Nazis from the old BNP and NF seeing the opportunity to take street violence back to the old levels, seeking a civil race war, this time white Christians against Muslims."

Could the attacks in Norway happen here? Collins nods. "The EDL's leader Tommy Robinson says himself we are five years away from a Breivik in the UK. If a similar attack was to happen here I wouldn't be shocked."

How does Collins think the UK should deal with this returning threat? "The last thing we should do is to listen to their demands. But we do need to deal with the issues on which they thrive: identity, alienation, the emasculation of the white working class, job insecurity."

He shakes his head. "Everyone's talking at them and not to them. Reaching out to communities, that is such middle-class bollocks. We need to embed ourselves in communities, take on the lies, redress the inequalities that grate on people. And we have to discuss immigration properly without demonising asylum-seekers."

In Collins' story lies the hope that even committed fascists can change, but it is hard not to wonder what the group of women at Welling Library, battered with chairs and iron bars, make of his change of heart.

At his book launch last week, I found the community activist Dev Barrah getting his copy signed by Collins.

"I'll never forget Welling Library," Barrah says. "29th June 1989." He was the local campaigner who had called the public meeting against the BNP and hired the room. They'd been worried about an attack but there was a Tube strike and the men they'd asked to be stewards hadn't been able to make it.

"I remember him particularly well," Barrah says, indicating Collins. "He was the loudest of them, shouting 'BNP, BNP', smashing everything. People jumped from the first floor window they were so frightened, breaking arms and legs. They came with hammers and crowbars, beating up women. After a while, we managed to lock all the children and elderly in the loos and still they were attacking."

He's standing with Collins' book in his hand. What does he think of him now? "I believe people can change," Barrah says. He shrugs, a broad, generous smile crossing his face. "A bad person can be a good one, too."

As the two men shake hands, it seems a terrible sadness that Collins spent all those years looking in the wrong place for heroes.