No way Christmas can be a busy time for WGs - Given that the UK shuts down for ten days the sensible WGs fly off to warmer climes to boost their tan and pull a few off the beaches.
You are right that the week between Christmas and New Years is dead except for family matters (and shopping) but WGs have family, too, so they are off on hols or doing the family thing, too.
I'd expect girls to charge EXTRA (OMG!) for service during the traditional holiday down time ;-) Supply and demand - market forces - if there is a scarcity of girls, the ones left might find a post Christmas flurry of activity (especially good way to spend the money cards from non-creative gift givers).
But the run up to Christmas is often replete with Christmas bonuses and a bit of self-direct generosity... afterall you don't expect your partner to give you a Punting Gift Certificate under the tree... and maybe the thought of being cooped up with the in-laws etc for a week means that a punter needs to sow his seed ahead of the family time ahead.
I surely shall be punting right up to the 23rd.