#blacklivesmatter racism is a human problem

Britain has seen continuous migration over thousands of years, whereas North American populations had little contact with Europe over hundreds of years. When the 'Celts' arrived in Britain 3,000 years ago (as is currently estimated), they will have been newcomers, undoubtedly not without conflict, to be added to by plenty of migration from throughout the Roman Empire (numerous finds attest to that), until the Saxons came, then the Normans, and people were very mobile in the Middle Ages:

In the 15th century approximately one in every ten people in England was foreign-born, and in London it was one in six.

https://www.bbc.co.uk/bitesize/guides/zyrymnb/revision/1

Despite the high level of migration then, it's still hard to compare it to the volume of migration into the Americas. Obviously, that, too, didn't start to happen at the press of a button, but grew over centuries; it was still more sudden than the history of immigration into Britain. Perhaps medieval immigration into Britain can be compared somewhat to immigration into the US during the 20th century, but European populations have always mixed irrespective of major waves caused by wars. We also know that there have always been many black Britons, long before Africans became the main targets of slavers. It's funny how creatures who evolved to walk upright on two legs have a tendency to move about the place. :)