Via several sites, a NASA photo of Great Britain from Thursday noontime:
The U.K. doesn't usually get a snow cover at all, let alone one this thorough. The U.K. Met Office has an explanation:
In most winters, and certainly those in the last 20 years or so, our winds normally come from the south-west. This means air travels over the relatively warm Atlantic and we get mild conditions in the UK. However, over the past three weeks the Atlantic air has been ‘blocked’ and cold air has been flowing down from the Arctic or the cold winter landmass of Europe.