The Met Office have published this account of what is causing this freezing start to the year. Basically, our winds in winter (particularly in the last 20 years) normally come from the south-west. This means air travels over the relatively warm Atlantic and so have brought mild conditions to the UK. But for the past three weeks theAtlantic air has been ‘blocked’ and cold air has been flowing down from Greenland or the cold winter landmass of Europe. Scientists call this contrast “warm-ocean cold-land phenomenon”.
The Met also note although the mean UK temperature for December was 2.1 °C, making it the coldest for 14 years and colder than the long-term average for December of 4.2 °C, December was one of only two months in 2009 which had a below-average mean temperature. In the UK, 2009 as a whole was the 14th-warmest on record (since 1914). This above-average temperature trend was reflected globally, with 2009 being the fifth-warmest year on the global record (since 1850).
Climate and weather ain’t the same thing.