In the arctic or the antarctic it's never too cold to snow, though much of it is ice, not snow. This is England, though, and it always warms a little before the snow comes. When it's really cold, not just wind chill, and there's a hard frost the sky is clear. When the snow clouds arrive the "blanket" they create warms the atmosphere slightly.
It's all got connections with the Gulf Stream and prevailing winds and other Geography stuff, most of which I've forgotten.
