A few days ago IanVisits published (as usual every month) a rather super list of things one can do out of London during the coming month — ie. December.
The list is full of wonderfully festive events including Christmas tree festivals, boy bishops, torch-lit processions, mummers, football and tar barrels. Many are (or are based on) very ancient traditions especially to do with mid-winter fire and light festivals. You can find the full list here. I commend it to you!