I live in Bridlington which is on the coast of East Yorkshire and believe it or not even when its snowing everywhere inland, we sometimes get nothing as we are in a bay
Nobody knows, especially not this far in advance!!!
Anyway, it may snow on Christmas Day in London but that doesn't mean that it will in Bridlington. I live 5 miles from my parents but because they are high up and I am lower and in a town centre, they sometimes get snow that sticks yet I won't see any.
I'm pretty sure for gambling purposes that London is the place that counts, unless specified otherwise when the bet is placed :-)