Don't think 15C is correct - looks more like 8.5C to 11C in England and 7C to 9C in Scotland.

http://projectbritain.com/weather/average.htm

You can look up ground temperature for your location here - https://weatherdownloader.oikolab.com/downloader.

Layer 4 is between 1m to 3m depth beneath the surface.

Apologies, I didn't appreciate you meant average temperature underground - that is definitely much warmer. We have a ventilation shaft for an old limestone mine near our house and on a cold day the warm air rising from it is very noticeable.

I wasn't the commenter so no worries about apologies... just wanted to point out where you can find this information. :)

Sorry, I was wrong indeed, thanks for the correction