Permenent separation is impractical and just stupid, however, I do wish immigration could be brought down to a much lower rate than it is now. I have no problem with qualified professionals immigrating to the UK, but the vast majority of immigrants are low-skilled, probably below average IQ menial workers. The UK does not need these people. Japan has the system that I think is most ideal, they pretty much only ever take skilled immigrants in. The least skilled people they take in are a few Filipina nurses, and nurses are still far more qualified than some toilet scrubber or burger flipper than the UK takes. Hell, you have to have a degree to even teach English in Japan.
I don't feel any reason to go into detail, but I find the thought silly. There's no real reason for "permanent" segregation (in my country at least), which I assume would be government-forced. Yes Immigration should have restraints, I'm not one of the real political heads here, but it's hilarious that people are opposed to multi-racial communities imho. Even considering many people self segregate to a degree (this is not always the case, nor does it need to be), I see no reason to forcibly do such. I have lived comfortably with other races my whole life, and believe it or not, I wouldn't change it. Unlike the majority here, I'm sure, I actually enjoy diversity.