Helping a local village hall improve their energy efficiency and sorting them out some internet, and found this. Do love a mystery...

It's even labelled unknown on the circuit documentation!

ALT

@jon @james I had one of those in my house but, after 20 years of ignoring the switch, I finally traced it to a box on the back wall this year, on its own main fuse. Which was incredibly fortuitous as I was about to get quotes for fitting an outside socket!

@jon @james
I had one in my kitchen - it had apparently been the switch for the immersion heater in the upstairs bathroom.
The clever electrician reused the circuit for my luxury Japanese style electric toilet (yes, really!)