glasnost said: Does the BBC broadcast subtitled in Japan? How in the world are Japanese people finding out about this show?
Tetrominon said: Nico Nico. Episodes/segments get uploaded there and then fan-translated with the comment feature.
Also, they have heard of the arguably more obscure show, "QI", if only for the wrong reasons, so it isn't much of a stretch if they heard of a more popular show.