I believe there was a strong El Niño around this period.

December 1918 had a CET of 6.9C

Dublin recorded a max of 15.6C on the 4th as did Scarborough on the 5th and Birr Castle on the 8th

The 23rd was stormy in the north, 72mph gust at Aberdeen.

Frosts did occur in the north but they were very limited further south.

Rainfall % of average

Scotland: 98
England and Wales: 153
Ireland: 139

Camden Square mean December temp: 7.6C