You say that but there are pockets of some large UK cities that are self imposing Sharia law around their largely Muslim burbs. This is clearly done outside of UK law. But it is happening, same as arranged marriage, child sex rings(Not saying Islam is responsible for this just some very reprehensible individuals inside of those communities) and very Islam centric schooling.
All of these things and the continued lack of integration of some of these increasingly large communities should be a concern to all. These communities can grow increasingly alienated from British culture, again this can be a product of relative poverty in the community and these are the areas that should concern us about extremism creeping in.
As others have suggested, I think poverty or perceived poverty and a consequent feeling of desperation, can play such a large part in the susceptibility of an individual to be radicalized. But then it's far from the only factor. The knob heads that killed Lee Rigby looked like on face value that they came from a decent neighborhood and family, so what went wrong there? An example that it is the minority who are unhinged enough to interpret Allah or any Gods will as murderous. I do think we could do ourselves a favour by keeping hate preachers out of the press and just quietly deport them.
To sum up;
A) Some people good
B) Some people bad
C) Being Religious
D) Not believing in sky pixies
So
A + C = generally good
B + C = Generally bad
A + D = Generally good
B + D = Generally bad
That's right, people are generally good or bad independent of religion. Religion is an excuse for some, but not the cause. The only danger of religion is that it can group bad people to do greater bad. But some would argue there are other means of grouping people to do greater bad - e.g. Political parties
Bah!