I don't know what thermometers are available in the UK, but try find one with an "outside probe", I find this to work better as you can lay the probe directly on the floor substrate; as you're trying to get readings for the floor temp and not air temp.
I'm currently using Taylor inside/outside thermometer.
I had got an accurite (wally world outdoor section). Has clock, outdoor probe, indoor humidity and a min max for temp and humidity. Problem is the outside probe SUCKS. swings + or - 5 degrees. No real rhyme or reason. The "inside" temp seems correct within .1 degrees to my zoomed digital so I use it for room temp and humidity.