Try to work out sometime who really is "British" you will become very stuck as our Islands have had immigrations throughout history.
from the Celts, Romans, Saxons and Normans onwards to citizens from parts of the old Empire and commonwealth to those arriving from Eastern Europe today. They all add something to the country.
Since when were the Anglo-Saxons, Normans and ect immigrants?. They were vikings. Stop trying to change history to suit your own backward need for foreign enrichment!
As for immigration, it didn't even rear it's ugly head in Britain until around the 1940's. It caused a lot of race riots in which old native British folk got stabbed by Afros and Asians!.
When one of my countrymen wishes to think about multiculturalism, all they have to do is think of Bradford and London. These two skanky cities are the biggest crime ridden slums in the entire western world!. Still, the countryside is still peaceful where the British still prosper. For how long is anyone's guess!