The Met Office publishes monthly historical data for a number of data-collecting centres across the UK.
I chose the Oxford station, converted the data to TSV and uploaded it to Timetric.
Filtering the time series to display a moving mean produced these graphs:
The full data sets for more recent observations are also available, but you have to register first, it's for research purposes only and it seems like uploading the data to third-party services might not be allowed…