The British convince themselves that they loathe extremism. Continental Europe experienced the devastation caused by fascism and communism in the 20th century, but Britain has not had a revolution worth talking about since the 1640s. In France, Marine Le Pen of the Front National may well be the runner-up in the 2012 French presidential elections as her father was runner-up in 2002. In this year's British local elections, the pathetic BNP managed to win just two council seats, while in the 2010 general election Labour took the sole parliamentary seat held by Respect, George Galloway's alliance of the white far left and Islamist religious right.

The success of the mainstream in vanquishing the fringe has reaffirmed a cheering stereotype. Dear old Blighty may not be the most exciting place on Earth, but it is a steady, sensible and, above all, safe country. Yet although extremist parties fail as badly in Britain as they have always done, Britain has become the European capital of extremist ideas. So much so that before you learned the identity of the terrorist who murdered the young men and women of the Norwegian Labour party, or the "reasons" for his slaughter, you could make an informed guess that Britain would loom large in his background.

The authors of the new and encyclopaedic "Islamist Terrorism: The British Connections" tell me that between 1993 and 2010, 43 individuals born, resident or radicalised in Britain are known to have committed suicide attacks abroad. As it turned out, Anders Breivik hated Islam in all its forms. No matter. The manifesto he left online showed that Britain is an equally inspiring source of ideas for murderous neo-fascists.

When I read it, I felt as if I was insulting the memory of the Norwegian dead. No one would have waded through his thousands of words if Breivik had not killed innocent people. Not even a fellow neo-fascist could have coped with his rambling pseudo-history or the 60 pages which Breivik modestly devotes to an interview with himself – apparently unaware that the announcement: "No one is prepared to talk to me apart from me" confirms him as a solipsistic loser.

Nothing about Breivik is as interesting as the people he shot, but those with the stomach to read him will find that ideas made in Britain enthralled him. He writes in English. He uses a British pseudonym – Andrew Berwick – and gives his manifesto a London dateline. He meets sympathisers in a London pub and drops strong hints that the organisation he is closest to is the English Defence League. He has a respect for at least some of the EDL's ideas because it does not go along with traditional antisemitic Nazism but agrees with Breivik that there has been a plot by the treacherous "cultural Marxists" of the European elite to undermine the nation state by flooding it with immigrants, most notably Muslim immigrants.

British extremists of whatever type have the advantage that English is the language of the web that foreigners must master if they want an international audience. Breivik's references to British sources would not be surprising if all he did was quote from obscure websites and chatrooms.

But Breivik did not only listen to British far rightists screaming out their hatreds in the madhouses of the blogosphere, but peppered his manifesto with citations of articles in the Daily Telegraph and other respectable conservative newspapers. Britain's mainstream media, not the fringe on the web, formed the basis of his claim that readers could find all the evidence they need of the multicultural plot to turn white, Christian Europe into a Muslim-dominated "Eurabia" in the "Neathergate" affair, a self-evidently absurd scandal that apparently serious conservative journalists and politicians have taken very seriously indeed.

The conspiracy theory began when Andrew Neather, a former speech writer for Jack Straw, wrote in the London Evening Standard that Straw and his Labour colleagues allowed mass immigration because they thought it would undermine the Tory party. "I remember coming away from some discussions with the clear sense that the policy was intended – even if it wasn't its main purpose – to rub the right's nose in diversity and render their arguments out of date."

Neather's account could not have been true, as he later half admitted when he said his views had been "twisted out of all recognition". Far from destroying white Britain by opening the doors to migrant hordes, Straw imposed ferocious restrictions on asylum seekers, which I and many others said stopped genuine refugees in fear of their lives finding sanctuary here.

I am not maintaining that the British right in either its neo-fascist or conservative form caused Breivik to commit an atrocity – a psychopath will always find reasons to kill. I am simply arguing that neither the British right nor indeed the British left cares what dangerous ideas they throw out, as long as they confirm the prejudices of their supporters. Not only on the fringe but also in the mainstream, the media ignore the Bengalis, Pakistanis, Somalis and Arabs in Britain who are trying to make a living like everyone else and constantly inflate the stature of extremists.

With a neat symmetry last year, campaigners against white neo-fascism from Hope not Hate wrote to the right-wing Daily Star to complain that it exaggerated "the importance of tiny Muslim extremist groups" and risked creating "a dangerous backlash among non-Muslims which in turn will feed groups such as the EDL and the British National party". Within weeks, liberal Muslims at the Quilliam Foundation complained to the vaguely leftish executives of Channel 4 that they, too, took speakers from Islamist groups and supporters of Iranian theocratic regime to "represent mainstream Muslim opinion" and reinforced "negative stereotypes of Islam to non-Muslims" as they did it.

Rightwing newspapers pretend extremists are immigrants' authentic representatives because they want to whip up the fear of the other. The liberal media think themselves daringly transgressive by giving platforms to reactionary and paranoid men because they want to revel in the exoticism of the other. The motives are different but the effect is the same. With a frivolity and ignorance of history few European countries can match, neither thinks about the consequences of pumping out the ideas of poisonous cranks.

But why should they? Unlike Norway in the Second World War, and virtually every other country in Europe between 1939 and 1989, the fascists or the communists never occupied Britain. It has never had a modern revolution or domestic dictatorship. It is a steady, sensible and safe place that never stops to think that being so stable is what makes Britain so dangerous.