British TV has been trying to cast more diverse actors for decades but, here we are, 2017 and it's still trying. It hasn't become a routine thing yet, it's mostly a sea of white faces, despite some stars like Idris Elba emerging along the way.
But at least no-one's given up yet.
Regarding interracial relationships in the UK; I can't really comment on the history, I'm sure there are studies by numerous academics, I doubt it was easy for couple in the 60's 70's or 80's and I think they've probably had problems from other people since then too.
Hopefully less than older generations did, but I don't want to get into a competition of who has it worse, UK or America. That's not a competition anyone should want to win really.
I'm going with the idea (mentioned up thread) that Jackson was talking off the cuff, doesn't know the UK that well (he's worked here a few times but its not his home) and would prefer to see his fellow Americans get the few lead roles for black actors that are out there.
It's not a big deal, just unfortunate.