G. F. Schleebenhorst said:
The origins of most white people in the UK are a bit of a mystery these days.
I'd only differentiate between indigenous Europeans and....everyone else.
It's not about black vs. white, it's about indigenous vs. non-indigenous.
Bearing in mind that this hypothetical situation would be set after I'd closed all the gates to more foreigners coming into my country, I'd make sure my country was as meritocritous (is that even a word?) as possible. That's if I haven't decided to send them all off to some tiny island somewhere.