Figures (for England this time) for all ages
from the 2001 census:
White 90.1%
Mixed 1.3%
Asian 4.6%
Black 2.3%
Chinese 0.5%
Other 0.4%
Immigrant populations in any country overwhelmingly go to big cities first (farm workers being the main exception, but immigrant farm workers to Britain have mainly been white), and it can take generations for them to spread out elsewhere; the mixture in London can be very different from elsewhere. For London, all ages, from 2001:
White 71.2%
Mixed 3.2%
Asian 12.1%
Black 10.9%
Chinese 1.1%
Other 1.6%