I don't know what your experience is of the places I mention but I know them well and I know what I've seen. It can only be called Islamification.
As for isolation the only people isolated are the indigenous population who would move out if they could afford to.
I actually enjoy wandering around places like Southall - there is a different vibe to other areas - I enjoy the diversity and I don't miss the scarcity of the 'indigenous' population. If every part of the country was the same it would be boring.
On the other hand I don't particularly enjoy the vibe of wandering around areas where people are very loud and aggressive, often due to excessive drinking. I'm not against all bars and clubs - some are pretty civilised and fun.
I've been to Nelson - once bought some property up there at rock bottom prices to rent out, but sold it all a while ago. Nelson was grey, boring and depressing the few times I visited - not somewhere where I would enjoy living, but I can't say that was because of Islamification.
What specifically was it about the Muslim population that bothered you?