I have a friend who's a sound recordist (worked on Waking the Dead and so) and she's often done 14 hour days (come to that, I've done 14 hour days, with another four on the commute home, for two or three days on the trot when coming up to deadline).
So really, the Brits don't do the overtime thing doesn't hold water: the legal limit that Sindatur mentions is a 48 hour-a-week limit under EU law, and Britain has an opt-out - so you can work longer if you sign an agreement saying you agree to do so. You just can't be forced to do so).
In any case, it wouldn't change the central point: there are so many hours that Matt Smith and Jenna-Louise can work and still learn their lines and get enough sleep to remember their lines.
That's the constraint that makes it difficult to make more than 14-ish episodes a year without changing the whole approach of the series, and turning it into 'Doctor Who and his team of six chums'.
It doesn't explain why they aren't still making 13/14 episodes a year as they used to do. That's down to other constraints.. budget planning constraints.