This is a fuse box, with a flathead you should be able to open it and/or pull a small tray with the fuse itself inside. You Brits do your electrical wiring with ring-shaped circuits and put a fuse in every outlet.
This kind of outlet is intended for things you don’t unplug and a socket doesn’t make sense, usually boilers, ovens, stoves, ACs, alarm systems as already commented…
There is, or was, tho.