What is a 'Brit'? Can you find one who's not a originally a Celt (=probably north European), an Anglo-Saxon (=German/Danish), or a Viking (=Scandinavian), plus or minus Norman (=French by way of Scandinavia), with maybe a touch of Huguenot (=French/Dutch/Belgian) or a bit of Irish (they did move around a lot)?
And that's even before we've done the Roman heritage bit (=Italian, plus all kinds of people embraced by the Roman empire, including eastern Europeans and north Africans).
Meanwhile, with the opening up of empires since at least the 1600s, people from Asia, Africa, the Americas and even latterly Australasia have also settled in Britain.
Going back a bit further, it's widely accepted that we (=all humans) originally
came out of Africa. So we Brits have that in our genes too.
That being so, why are some people in Britain so unwelcoming to 'foreigners'? We all have a bit of 'foreign' in us. And in fact it's pretty well impossible to say which bits of our DNA came from where. 