I believe the BBC used the theatre showing for a number of shows. It enabled real audience laughter to be experienced at the right places and in proportion to the funny moment. I think the indoor studio scenes were actually shot in front of an audience, rather than being shown to them on a screen (which of course HAD to be done for location shots), which allowed the actor to delay their lines until the laughter from the preceding funny moment had died down. I guess this is part of the skill of timing that actors and comedians have to learn. You can see this latter phenomenon taking place on every show, during the indoor sequences, like in the café and in the houses of the characters.