When trying to create something representative, it often gives a better result to over-represent than exclude, because the latter may imply there were none.

I'd argue a large reason we're having this discussion at all is exactly because the idea of black people in Britain at that time is surprising to a lot of people, exactly because a lot of people have an image of the Romans as being mostly white Italians to the point where seeing a black character at all is a surprise, rather than out of concern that they're not accurately representing the precise proportions.