I think a British accent is associated with sophistication, culture, and intelligence.
It's kind of funny but if you someone to sound really smart in a movie you give them a proper British accent. But if you want someone to be really nasty and evil you can give them a British accent and that works as well.
Because it doesn't sound as harsh as the American accent. Not sure why but when Brits get angry not the same as Americans...it's all about the language signals that are either intimidating or not.