This data was compliled by Simon (known by the popular name of GW)

Ok so here is the data using 30 year averages that roll forward every 10 years starting from 1701. So for example 1701 is based on averages for 1671-1700 (and so are all the years up to 1710 before the average rolls forward by 10 years. 1801 is based on averages for 1771-1800 etc.

What you can see is that 1879 is the only calendar year in which every month was below average.

The longest uninterrupted run of below average months is from the same period. It is 15 months from November 1878 to January 1880.

The only other occasions where there was a consecutive 12 month run of below average months were November 1798 to October 1799 and July 1950 to June 1951. That last one is a close call because 4 of the 12 months were less than 0.1C below average but technically were just below average when you go to two decimals.