Over the years i have seen lying Snow anytime from November to April,however,these days with warmer seas around us a Northerly rarely brings a prolonged snowy spell.Therefore i would have to say late January to end of February would be be best for an Arctic blast for the UK when the sea is almost at its coldest and the sun still not too high. As for Easterly`s then i guess the window of cold is larger say from from December to early March as the landmass to our East can cool much quicker earlier in the season and ,with right sypnotics, can still be quite cold as late as early March. Unfortunately in recent years the right sypnotics are less common,certainly compared to my younger days of the 60,s and 70,s.