You mean in the tag scene? It is, of course, the real set of her flat – which is what you’re objecting to. But it’s also why they did it. As I say, it’s a very self-referential episode.
But I don’t think that trick is as much of a problem indoors as in the street outside her flat: because again the same set is used twice – but in this case, in one instance it’s actually supposed to be outdoors and in the other it’s on a soundstage. I think it annoys me because it’s shameless. So much of series 5 (to date) has used (bad) studio sets for outdoor scenes (see the fake woods in the previous episode for example); it’s a real step backwards from series 4 which featured lots of real location footage.