there are no identifiable "Islamist communities" in Britain

Yes there are. MI5 and the police are aware of many.

and no Muslim community would ever commit or condone such an act.
Not the majority of ordinary, decent British Muslims.
Apart from the public pronouncements, there were many who protested and showed their disgust spontaneously.
I remember those with placards saying "not in my name" after 7/7.

Only fool (you Jim) would deny that there is a significant minority who believe in Islamism.