As well as the Saudi opposition, Britain plays reluctant host to militants from Tunisia, Bahrain, Egypt, Algeria, Yemen and Syria. The government is happier to host people opposed to the governments of Iraq and Iran, neither of which is seen as a friendly country.
But the latest arrival came from a British ally, the Gulf state of Bahrain, where a majority Shia population is ruled by an unpopular Sunni Muslim dynasty. Sheikh Ali Salman, a charismatic preacher in his thirties, was a leading figure in religious opposition to the government until his deportation last January. Now he and two fellow clerics have joined a broad range of secular and middle-class exiles calling for constitutional reform and an end to British security support for Bahrain.
Another prominent figure is Rashid Ghannouche, leader of the banned fundamentalist movement in Tunisia. His presence in London has drawn bitter criticism from Tunisia's secular government.
London became a centre for the Arab media after civil war broke out in Lebanon in 1975. Arab governments liked to subsidise publications to write favourable prose about their own achievements. Conversely, they also enjoyed financing writers and commentators who would abuse and undermine their foes.
It was all very tortuous, esoteric and insignificant. But with the emergence in many Arab countries of a new form of opposition, the Arab media and the exile scene in London became a magnet for dissidents.
The old dogmas of Arab nationalism and revolutionary socialism on a Soviet model are dead. The modern opposition movements are, by and large, "Islamist", that is to say they consider existing Arab governments illegitimate and call for the establishment of a political order founded upon the Koran and sharia law. Amplified by the Arabic and Persian services of the BBC, which are listened to with great attention throughout North Africa and the Middle East, the babel of criticism from London reaches a vast audience. Arab governments, in response, are increasing their own influence. The BBC's Worldwide Television is contracted to supply Arabic news to a Saudi-funded satellite channel, although its editorial independence has been guaranteed.