^I'm not sure Allyn has it right about the hours they work. I remember them saying 16 hour days, all night shoots etc. So the no overtime thing probably isn't true. The other thing is they shoot far more on location than most shows in the US and only a small amount of standing sets, and have only a few stars no ensemble like most US shows. I heard a quote saying that British Actors are getting a rep as the Mexican workers of Hollywood, they'll work harder and cheaper than US equivalents, so it can't really be that British schedules are easier.
Well I'm certain about how they film the new show but on the old series they'd rehease all week and shoot the ep. on Friday which would take about seven and a half hours. At least that was true of the show back in the '70s according to the text commentaries.
Now I don't know how much reheasal time they get on the new show, but it was a major part of production on the old series and I'm sure that extends the time to produce a ep. even for the new series.