I don't know the racial history of the UK, but understanding it (as a layman, not with a degree) in the USA I'm not surprised that interracial is so much rarer (and also why it's easier to accept a white man with a minority than say a black man with a white woman). Long story made VERY short, the US slave trade messed up in trying to stamp out the cultures of black people brought over which in turn created a relatively united black culture with shared experiences (though there are odd exceptions, like say Louisiana Creole, but I'm not up to going into that now).
I'm not even going into the romanticization of the Confederacy which was fought over slavery rather than state's rights (as shown in their own writings and constitution) other than to say it aggravated the situation because the end of slavery was a vivid reminder of their loss, and thus the violent racism and oppression was exacerbated in this region. (This doesn't mean everyone who has a confederate flag is racist, mind you, and many Cherokee--which included slaveholders I might add--fought with the Confederacy, in part because they hated the USA over the Trail of Tears. Likewise, there were Southern whites opposed to slavery even before the Civil War, but if they acted on their beliefs to rectify the situation then they were considered criminals at best. And as for today, while the psychology is complicated, I think it's unfair to automatically assume a Southerner is consciously racist, white or otherwise, though they can be found there as anywhere else.)
There are other factors like the fear of what happened in Haiti happening the USA (because on some level, I think many whites knew they deserved it--not that I think it should've happened as that's becoming the demon you set out to fight, though I wouldn't promote silence and pacifism either). This created court decisions and got laws passed (and rather than dogwhistled as today, were blatant about how it was intended to disenfranchise and disempower the black population in particular so that what happened in Haiti didn't happen in the USA), though it got increasingly subtle through the 20th century, it remained brutal and horrible to at least the 60s, and I'd say 80s at which point dogwhistling (that is continuing to step on the necks of minorities, especially the feared black population, continued, but now happened under code phrases and gaslighting rather than as open as it once was). Many of the same arguments against gay marriage (including Biblical ones) were used to justify laws prohibiting interracial marriages back then, though that eventually collapsed, something some people still alive remember and have brought up.
Though this happened all over the USA, there are still people alive with the fear (sometimes conscious, sometimes not) of karma and of old attitudes passed down. Heck, I know a white guy who was nearly strangled (as in rendered unconscious and unable to swallow right for years) because his dad caught him, as a minor, shaking hands with a black man who helped him--imagine how that father would be if his son dated a black woman! (This doesn't count casual sex--at least not when it's a white man with a black woman, but many white men are insecure with a woman who has been with a black man. And even here there's a lunatic fringe, like the KKK, who believe things like it's actually corrupting to touch someone black, perhaps that abusive father was the member of such a group, or raised by one.)
When this is a problem, the symbolism of a black man with a white woman can seem pretty threatening even if the viewer isn't sure why, and those with conscious racist attitudes (who are distressingly numerous) also don't want to see such things normalized (even if Kirk and Uhura are forced to kiss, if it's a one time thing under mind control, they might give it a pass, but they don't want their kids to think that it's actually okay).
While I don't want to expand on this, a sexual assault I endured at the hands of a black man that got ended fast before I was hurt too badly actually made some people more angry than a much worse attack by a white man that left me scarred with a host of other physical (and mental) problems. Interesting enough, when I asked why (in this case I wasn't just curious, I was angry at the implications) even they seemed to not realize why. It was just down in their subconscious (much like how I was scared to leave my tomboy ways and didn't even realize it until a girlfriend wanted me to femme myself up for her, and THEN I realized I was scared and had to explore my subconscious assumptions that were guiding my reactions and emotional responses without my conscious awareness).
And if it makes them (for conscious or subconscious reasons) change the channel or avoid a movie then it's not going to be done much because profits rule all (though there have been exceptions, and kudos to those who allowed their station in the 1940s to be used for a radio program of
Superman fighting the KKK which had a positive effect on future generations since so many kids, even the children of Klansmen, took Superman's side).
I do see it changing today. While sometimes it comes off as cynical and insincere pandering (that is they'll have it happen in a show, but if the kids of the writers and producers were to get into an interracial affair then real life drama would ensue, and is written in such a way that it can annoy people for other reasons than it involving interracial couples), some of it is working out well. I personally like how the
Chilling Adventures of Sabrina have pulled it off (though sanitized and "modernized" from the comics) and I know
Stranger Things had a black boy with a white girl (but I haven't seen it yet) which I haven't heard people complaining about despite how popular it was and that it presumably taps nostalgia (in fact, sadly enough, I've heard more Buffy fans complain of Giles with Olivia). Like so many other trends, it is going to be diluted every generation until it's no longer seen as an issue by anyone who isn't against the media anyway.
Again, I don't know how it happened in the UK, but I do know its history is very different from ours in this respect, so it doesn't surprise me that it gets reflected in their media, probably because it doesn't stoke the same anxieties (and thus threaten ratings/ad revenue) as it does among significant numbers within the USA.
