Because I work for a British company and hear Brit accents all the damn time, the accent no longer sounds exotic to me, but I suppose that's why it gets used as an advertising tactic.
BTW, I spent a week working in a small town in southern England a few years ago. I was on site with a customer, a large British "building society" (like a credit union), so I had virtually no exposure to Americans or their accents except over the phone when talking to coworkers back home. After the work week was over, I spent the weekend in London sightseeing. I was walking down a busy street in London Saturday afternoon and overheard some Americans talking behind me, the first I had heard not via phone all week. Their accents were jarring to my ears (prob midwest or mid atlantic). I remember thinking "this is what we sound like?".
Chris