But total levels of immigration are different to multiculturalism - We already are multicultural, and we can either like it or spend our lives getting annoyed because the local library has posters up with some Urdu writing on, or because we can see a Mosque from our house, or even because there's an Italian restaurant nearby. Personally speaking, I can't think of any harm that living amongst people of other cultures has done me, but I guess we all have different experiences.
With regard to when the UK is full, I don't know, and I suspect no-one else does either but at the moment I would say we were poorly distributed rather than overcrowded, and that could be solved in a number of ways. Holland has over one and a half times the population density of the UK and that feels fairly spacy to me too. We are at the top end of European population densities, but being sparsely populated isn't necessarily a good thing either.