It doesn't work because immigrants naturally move to places with more of their own nationality, rather than being spread out across the nation. It's always been like that AFAIK.
This leads to; rather than being readily integrated into British society and culture, it is now more common practice for immigrants to continue their cultural practices in our country. My girlfriend recently worked somewhere where one of her colleagues expected her to open the door for him, he would stand and wait at the door for her to open it, and not do it himself. It is strange little things like this that seems to have an impact on people the most... it is our culture for men to open doors for women, not the other way around and that is where the clashes can begin. Scale that up to religious, political and social practices, all competing for space on our tiny island and it is asking for trouble.
It is British culture that dictates our acceptance and tolerance or practices of others, however with social media, blogs and the internet allowing people to voice opinions without being publicly bombarded with 'racist' and 'it's people like you' comments that will stir an uprising. Anonymity. There is already a horrible undercurrent of extreme views and opinions about immigrants in this country (BNP, EDL etc), and it is the same throughout Europe. I hope it doesn't come to it but I would not be surprised if there were race riots within the next ten years...
Personally I'm happy for people to practice their religions, cook their cuisine, marry into their own ethnicities if they want to and generally be themselves in Britain, but things like honor killings (rare), un-equal attitudes towards women, street gangs and being armed are all imports which should be stamped out here. What I also find disturbing are racist attitudes between immigrants, of which there are more than people realise.