I don’t mean to be a prick, but maybe I sounded like one. Still, I don’t think the claim that black people are underepresented in British movies is valid.
You are right that I probably overexaggerated the extent of disproptionate numbers. However, it still seems to me, the numbers of roles black people play outnumber their part of the population. So, out of say 100 movies, there are maybe or about leading 300 role parts. According to this number then, there are 13 parts out of a possible 300, which still means that black people outnumber their percentage of the population. the 5% figure you mention isn’t that correct? Considering they represent 2% of the population?
It isn’t easy making constructive arguments about race, and in different contexts I feel my arguments may fail. However, it is hard for me to imagine a world where we can say “a group of 2% of the population, only plays 13% of leading roles in movies and is therefore grossly underepresented”