I'm guessing some media article sensationalised that and claimed it was abnormal and due to climate change?
There are issues with some of the data, as they have been 'adjusted' due to instrument changes or site moves, and the tendency should be for many to measure warmer temperatures as buildings get nearer, but even allowing for all that, 9 degrees is pretty much normal. Select some other years for comparison, and you could argue it's a sign of cooling.
https://www.metoffice.gov.uk/pub/data/weather/uk/climate/stationdata/heathrowdata.txt