Well, I think it's because the British accent makes the voice sound deeper - James Marsters, Spike from "Buffy the Vampire Slayer", managed a very convincing British accent throughout the show, but when he went back to his normal American accent on "Smallville", his voice sounded higher.
I remember hearing a comedian explaining why guys like girls with raspy voices: "They hear a girl with a deep, raspy voice, and then think 'Wow, she sounds like she's all done yelling.'"
Apparently we Canadians have an accent, too, (the whole "aboot" joke - although that's kind of exaggerating it), not that I've noticed. I've always wondered - are people capable of recognizing their own accents? I never thought Canadians (other than Newfies) had accents until the "aboot" jokes...