I recently implemented a simple directory page on d8taplex which lists the sites from which data has been extracted. Looking at data from The Commission for Rural Communities I found this time series data set on the thermal growing season.
The thermal growing season length is defined as beginning when the temperature on five consecutive days exceeds 5˚C and ending when the temperature on five consecutive days is below that threshold.
[See wikipedia for another, similar definition.] A simple - and reasonable - interpretation of this data, then, is that things are getting warmer.