I didn’t say do a British movie, you keep giving examples of American movies with American stars doing accents, I just told why the accents are bad, they pick a star over “great accent”
And I’m coupling African and brits because the will smith example you gave was an African accent, therefore per your premise, a Brit would be better because of “how great their accents are” or an African (Nigerian) since it would be their actual accent. Secondly a lot of black brits are immigrant or 1st/2nd gen Africans