Sorry, and to clarify two points.
One is, forced marriage is cultural, not religious. In Birmingham it's a problem that schools are trying to address, but it is certainly not restricted to Muslim girls. It's also happening in nominally Hindi and to a lesser extent Sikh communities. The common factor is conservative, less well educated and less well integrated first generation immigrants from quite small geographic areas, not religion. The desire to "marry in" to preserve a cultural heritage in a "foreign" country is hardly unique to any one religion.
Secondly, when I gnomically said "what makes you think they don't", I was unfortunately looking a bit dog whistle-y. The point is that everyone reads newspapers or sees the television news. Muslims in Britain "know about it" in the same sense that I, as a white Briton, "know about" Fred West. As Eldridtch has pointed out, it is flat-out racist to say that people from group X should condemn every bad thing that other members of group X do, even if they claim to do so in Group X's name. It's McCarthyite, imposing a loyalty test on people just because of their ethnicity. Of course, the left is not immune to this - there's a tendency to demand that Jewish Britons have a position on Israel/Palestine, which is just as racist as the assumption that Muslims who don't start every conversation with a long list of condemnations are somehow friends of the Taliban - but it does seem that "othering" Muslims on the grounds that somehow their silence on forced marriage (or genital mutilation, or the veil, or whatever) is equal to support is a new, and nasty, tendency.