OK Justinian, I went to London today. Predominantly, its whites. Of course, if you go to Camden, the shop owners are of ethnic origin because a lot of the market is themed around that.
The reason there are so many blacks/ethnics is because its one of the central cities of Europe, and the Western World, so it attracts tourists, business people and government officials etc. from all over the rest of the globe.
The only trouble we actually got into was with whites actually. Punks in Camden. My friend is a punk, and recently got a nicely even cut moheikan, and wasnt wearing a big pair of steel toed boots, so they accused him of being a poser punk ('****ing weekender was the phrase they used I think). A lot of the punks in the area, ironically, are neo nazis.