The internet will always have proof of the day the Doctor checked out Sherlock. <---- Haha, don't be embarrassed, Matt! I'd totally check out Benedict Cumberbatch too if he were standing right in front of me!
It's because the tardis translates into your native language, so if the speaker is not speaking English, it defaults to a British accent. This also explains why Americans and jack Harkness do not have a British accent.