JA
Though I'm sure if they really wanted to they could have filmed the models and played the recording rather than doing it live, so would have been able to keep them on air for longer.
At the risk of sounding like a broken record ( I seem to point this out every year) - the BBC were restricted back in the day in that their Christmas idents were physical models that were set up in Pres B. Tying that studio up for a few days over Christmas was ok, but using it for any longer would have caused problems.
Though I'm sure if they really wanted to they could have filmed the models and played the recording rather than doing it live, so would have been able to keep them on air for longer.