Can anyone identify this mystery outlet?
That is an older style UK "unswitched cable outlet", also known as an "unswitched flex outlet". The current style have a cable exit point at the lower edge, not the middle of the faceplate. They are typically rated for 20A.




It is used for permanently-wired electrical-loads such as heating systems (boiler, immersion heater, etc). It is likely some device was removed from inside that cupboard and the cable disconnected. It might be a dedicated circuit with it's own circuit-breaker in your main panel ("consumer unit").