All BBC America shows are British shows, and the announcers are all British as well. It's just not the same exact programming they show in the UK. I suppose that commercials are American, but I've barely watched a commercial since getting a TiVo, so I couldn't tell you for sure.

In fact, a quick scan of the real BBC schedules shows quite a few American shows, so you're probably more likely to hear an American accent on the real BBC than on BBC America.