There really aren’t that many British superheroes in Marvel or DC, period. Wikipedia only lists 23 British superheroes total, ever, from all media. That’s obviously an undercount (they don’t list Marvelman/Miracleman, probably the single most prominent UK superhero, for example), but still, it’s a pretty small sample size. Marvel and DC superheroes are overwhelmingly American, and extraterrestrials probably outnumber non-American humans. Add to that the fact that for decades Marvel and DC superheroes were overwhelmingly white, and the intersection of “Black”, “British”, and “Superhero” is unfortunately going to be pretty small.
Add to that the fact that in the American imagination, at least until very recently, the UK has been very much a White country. Heck, in the British imagination, until fairly recently, it seems like the UK was a pretty White country. In the last couple of decades, British TV and movie exports to the U.S. have had increasingly diverse casts, but for most of the history of Marvel and DC comics, their writers and artists would mostly have been consuming pretty darn White depictions of the UK.
In the DCEU, I’m trying to think of any other British metas, of any description, and…Gaius Grieves, also from The Suicide Squad is the only one I can come up with.
In the MCU, Karl Mordo from Doctor Strange is played by a Black, British actor, and is presumably supposed to be a British character. Beyond that, the only other British MCU meta of any description I can come up with is Captain Carter, from What If…?.
The MCU’s Asgardians have British accents, for some reason, and, interestingly, Idris Elba plays Heimdall in the MCU, so he may be the only Black meta with a British accent in both in the MCU and the DCEU.
It’s also true that there are a lot of British actors who have prominent roles in the MCU and the DCEU, but very few of them are Black, and a lot of them are Idris Elba.
*By the way, technically, Bloodsport in the DCEU isn’t a meta, he’s just a highly skilled killer with high-tech weapons.