What does it matter at the end of the day? One person with African ancestry of any kind is not significant in terms of the ethnogenesis of the British people. That's why it's not a big whoops.
As to whether 3% coverage is enough, I'm not sure. I do know that African ancestry is so divergent from that of Europeans that it's usually very obvious.
Egyptians, Saudis, Levantines, definitely tend to have East African ancestry, not West African. However, Moroccans are a different story. There are Moroccans with up to 60% SSA. Could one of them have made his way to England? Why not? There was still some trade even if nothing like before the fall. In the 10th century Ahmad Ibn Fadlan wound up among the Volga Bulgars and the Vikings. Would I be surprised if he left some offspring? No, I wouldn't. Fun, fictionalized movie about an Arab traveler among Vikings is called The 13th Warrior. :)