Purely for the London area the probability for Xmas day is skewed to being mild. I've taken a perhaps foolhardy stab based purely on events since 1961 - less than half the data I used for the winter forecast. I've posted full methodology again for your perusal. Because it is a single day - as opposed to a 90 day season - it is just a bit of fun really

http://wp.me/p2VSmb-iB