I found a mystery socket thing in my cupboard and I was wondering what it is. Here are some pictures of the front and inside the box.
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").
That appears to be a "plugless" plate intended to connect a flexible cord (as is usually attached to a plug) semi-permanently to power. Of course, there is no such cord connected at the moment.
The cable clamp on the opening and the extra screw terminals to connect the cord conductors to are the clues, here, after seeing that nothing "plugged in" would connect to anything, but that it's clearly well-grounded and keeps the power connections nicely isolated.
The face plate is the same as that used for a coaxial cable that would run to an outdoor/roof aerial or a satellite dish. However the wiring is UK Mains (AC 240v) & fairly old as it still has the red (L) & black (N) coloured insulation. The wires to your outlet are not part of a ring main but are or were a permanent live feed/spu3ee possibly to a storage heater or similar as looking at your photo, the cable guage isn't thick enough to carry more than 20A safely. That rules out higher load appliances eg: cookers, showers or immersion heaters etc which at 40A - 45A would overload the circuit, (serious fire hazard). Also I don't see any brown scorching at the terminals or other evidence of over heating. It should run from the supply via a consumer unit or other isolation point - most likely an old fuse box given the age of cabling. Provided there's no current running to it and you can't locate any point supply side it's likely just been left simply due to being out of sight. If the property was ever owned by your local authority and they carried out any electrical work, it was accepted as common practice not to remove sockets or obsolete cableruns so check the property history if you don't know. Finally, check if other wiring in the building complies with the newer colour code Brown (L) & Blue (N). It may help you.