One reason is because Islamists have been active for decades and because of soft multiculturalism and cultural relativism it wasn't challenged.
Another reason is that British Islam is defined by Pakistani Islam because Pakistanis are the largest Muslim group and Pakistani Islam has some very insular and extremist interpretations.
Add in Salafi / Wahaabism and you get the picture.
Yup, basically the source of the Islam in the UK; Pakistani not Turkish as in the Netherlands and Germany; and the pussyfooting of society around Islam (saying its definitely not bad and calling criticism of the ideology as Islamophobia). This joined by an overriding political correctness in the UK about everything with everybody seeming to be so damned sensitive about 'racist language' creates a melting-pot of hush-hush and lack of opposition to Islamic issues.
To be honest with you I think that people should be offended from time to time to keep them questioning of themselves, and also if people are not allowed to say the unsayable then resentment builds increasing the risk of huge civil unrest eventually. I don't think it is very healthy for a society to take notice of comments made by people like Jeremy Clarkson saying the word nigger. To be honest with you if black people don't like the term they shouldn't use it either and should encourage black people not to use it.