Santa Con apparently started in San Fransisco in 1994 (via wikipedia). Basically this is just a great opportunity to meet new people and participate in a pub crawl with your friends through large cities dressed as Santa.
Google answers the question…
What is the point of Santacon?
SantaCon is a bar crawl in which participants dress as Santa Claus or other Christmas characters. It is also known as Santarchy, Santa Rampage, the Red Menace, and Santapalooza. The event is described by its organizers as “a nonsensical Santa Claus convention that happens once a year for absolutely no reason.”
While waiting for the tube we stumbled across some locals dressed as Santa and asked if we could tag along. They were so gracious to let us join in the fun. I found out that Santa’s in England like to chick brussel sprouts at others, I guess it’s better than coal?