Welcome to Flyertalk Chatogal and welcome (finally!) to the BA Forum. Please continue to post here, and have a good look around, I'm sure you will find this useful. Some answers to your questions (not all of them) can be found via the Forum Dashboard, so that's a good place to check.

If flying Club you can go airside from 05:00 hrs, if you feel so inclined. The spa treatments are free, but in previous years the Spa either has been closed on Christmas Day or not open for very few hours. All the lounges will be fairly quiet, I would go the South lounge first however, or perhaps to the one in T5 B gates. As for Christmas theme, there may be turkey on the menu and perhaps some cabin crew wearing hats from their crackers, but it's not exactly overdone.