Both the groups I'm involved in play at pubs where we don't pay to use the space, but at this time of year they often book out for (paying) office parties, so things do tend to grind to a halt a bit during December. I didn't play last night, for instance.
Also, London being the sort of place it is, a lot of people head off out of town over the holiday period to stay with their families in other parts of the country (or even other countries), and that can make it difficult organising one-offs over Xmas. I'll be going to Canterbury, for instance.