Using data from the UK Met, May Tmax (mean maximum temperature) in the UK is cooling at -.618C / decade for 15 years.
The warmest May Tmax (in the last 104 years) was 16.9C in 1992. May 2013 was 13.8C. Tied for 33rd coldest out of 104.
CO2 seems to not work in May in the UK ….