If you are prepared to venture south if the river you will find the prep schools v diverse, in Clapham and Dulwich, as Tea says. And despite another poster's views, the Dulwich senior schools are ethnically diverse too. I am afraid you get teased a bit if your hair is red, but I know of no other prejudice, indeed the kids are very anti prejudice and you only have to see the teams, activities, trains and buses around to realise that.
She might be the only person with her particular background, but in DS's class there are lots like that. 1 Ghanaian, 1 Nigerian,1 Pole, 2 Czech, 1 Korean, 1 Malaysian one or two Afro Caribbean, ( Grenada I think, and Jamaica), 1 Macedonian, some British/caribbeans. 1 Spanish, etc. with masses if different bits added in.
we worked out last year that out of the class, only four had both sets of grandparents born in the UK. And of those, one set came from a former colony, and one set from Russia. And none was " pure" English.