I'm not sure how true this is any longer; I was recently (about a week ago) listening to a BBC radio broadcast on NPR, and noted how they were reporting in Britain (on something or another), and there was an emergency vehicle siren in the background - that sounded like an "American" siren! I've noticed this on occasion in the past, as well; I'm not saying they all sound like this, but maybe a transition is occurring, or something else (then again, I have heard the opposite as well - I'm not sure what is going on)...