the met office's definition is something like at least a single snowflake dropping on the emt office roof, what about if london is the only place that doesn't get snow and everyone else gets snowed in? (or is it any met office station across the country) Plus from their definition, it wouldn't count if their was a heavy snow storm on christmas eve, which means there is inches of snow on christmas day?
looking at this, it seems that you would bet on an area and the met station in that area is where the snowflake has to fall to confirm a "white christmas"