Looking at my records, dating back 5 years, shows this to be the warmest December by a massive amount.
Mean Dec temperatures, 2011-15: 7.2, 6.0, 7.4, 6.5, 11.1.
We took delivery of the weather station 5 years ago today and December 2010 was much colder than any of those. The 10 days I recorded averaged 3.2°C but the preceding 3 weeks were much colder. We had a heavy snowfall about a fortnight before Christmas and the temperature struggled to get above freezing until Boxing Day or thereabouts.