Here's an article I linked to on that other thread Azizi mentioned - The First Black Britons

It's not true that there weren't a fair number of black people in Britain during the slave trade days - but there wasn't any plantation system, or a separated-off culture over the generations. With the end of slavery, descendants of black slaves and freed slaves mingled into the rest of the population. The number of people in England with some black ancestors is probably very high.

Some of the major ports have a different history, in that there have always been a fair number of black sailors over the generations, and as has been noted, the shanty-singing tradition reflects that.