Very few of them are actually Madrassa trained.
The problem is simply this: Islam is designed to be a combination of a religion and a form of government. For small numbers of expatriate, non-devout Muslims, the mosque is simply a place to get together and pray like they did in the old country. For larger numbers, the mosque becomes a civic center, almost like a cross between a church and a city hall.
In most Middle Eastern countries, this doesn't cause a problem, because the overall government doesn't really care what the mosques say so long as they keep their criticism of the dictator silent. In fact, the mosques help keep society organized, which is advantageous for a corrupt tyrant.
In Western countries, however, there are no secret police coming to shoot the Imam if he says "Chirac (or Blair or whoever) is a craven infidel and it is the duty of good Muslims to resist him." That means that the truly vicious versions of Islam, the ones that are kept at bay by gunpoint in the Middle East, are free to run wild.
What we are seeing in England is that the power of zealots can be curbed only by keeping their numbers low, or by keeping a lid on the mosques by way of a brutal, repressive policing. Otherwise, they will fight day and night to raise a Taliban style government wherever they are.
"Otherwise, they will fight day and night to raise a Taliban style government wherever they are."
Quoted for accuracy. The real lesson in all of this.