Quite honestly Ricky Whittle was best for this part. Nothing to do with him being British. A lot of American Black actors are more concerned with #blackexcellence and a fear of black women than they are with diversity and getting good roles. The thing is you can stereotype some of these american black actors out here because they will put up with it, hell even do it themselves. Actors like a Idris Elba, John Boyega, Ricky Whittle, Chiwetel Ejiofor, Roger Cross, ect. can't really be stereotyped. They are free to all roles without much backlash. Take American actor Chawick Boseman for an example and look at his film history, (btw he is starring in the new film about Thurgood Marshall later this year) he has a lot of diversity in his roles but has also played the "black" role, but as soon as he gets a role where he'll have a white woman as a lover (especially since they now see him as Black Panther) he is going to catch hell. Ricky Whittle will not. The only american black actor I can think of that seem to have bypass black americans limiting their role diversity is Mahershala Ali.
Unless they would of chosen a American Black Actor that could get pass the limitation, the show would not be doing so well and there would of been groups out there trying to take it down because the main character doesn't have a black female as a love interest and he is not "black enough".