I think one of the reasons for having the different voiceovers is probably due to regulations regarding the amount of imported programming they can show and they may be able to bypass that by using British voiceover actors.
I've noticed this on the History Channel and Discovery, where documentaries that are of obvious US origin have British voiceovers.
A few years ago a friend in the US sent me a video of the History Channel series, Founding Fathers, as I am interested in the American Revolution, and he'd told me about this and I kept checking the listings and eventually emailed History Channel UK and they said they had no plans to show it. So I watched that with, of course, American narration. A year or so later it finally came on over here, and had an English narrator....but just comparing the intros, there were distinct differences in what was being said and the angle that put on the documentary! I think I still have both versions on video.