What makes you think of them as sinister?
Hollywood had a lot to do with that, I’m sure. All my exposure (until about 7 years ago) with accents from your side of the pond came through entertainment. Tim Curry claimed the only roles English actors could get in Hollywood were villains and butlers. And I don’t watch things that make use of butlers.
Although, that quality of forced that you identified is probably a factor in sounding a little sinister. Like a pretty facade hiding something ugly.