From Accuweather:
The central England Temperature (CET) from the 1st-7th of December is -1.9, making this the coldest opening week of December since 1879; 1879 is the coldest opening week on CET record, so this week has been the second coldest opening week to December since CET records began in 1659.
The two-week period, last week of November and first week of December is the coldest since CET records began in 1659.
H/T Sara Chan.