There is a far bigger representation of Black West African and Caribbean actors in Britain than other ethnic groups like Chinese, Indians and other ethnic groups. If you consider other non British actors from Europe the case is the same for them. The advantage in America for British Black Actors is the UK has a more accessible education system. The education system is cosmopolitan and looks outwards and not inwards.
An even bigger problem in Britain is a class system based on private schools and Universities like Oxford. Working-class Brits have enormous obstacles to overcome at all levels.