Muslim fanatics turning some parts of the country into no-go areas would be of concern definitely. Has anyone come across one? I haven't yet, and quite happily walk around where I live (South London) in very high density muslim areas.
Of course, there will be muslim areas that are inadvisable to walk around on your own at night, but then the same could be said for the estates round my way, and it wouldn't just be asian kids, or muslim kids, it would be white kids, christian kids, black kids and anyone else who decides to hang around hurling cans at pedestrians and spitting at them.
My point is that there are no go areas everywhere (and it has always been that way), and it needs to be tackled without singling out one particular community.