I can't work out what this switch is for
In the UK, switches with fuses are normally used for high-power and/or permanently wired appliances such as heating systems, electric-showers, ovens and so on.

It is possible your consumer unit (AKA distribution board or fusebox) has a circuit labelled for that outlet. I would check that and try to match up every circuit. 

A normal small UK home might have


downstairs ring-main
upstairs ring main
downstairs lighting
upstairs lighting
central heating
kitchen (electric oven, hob etc)
electric shower
garage


If there is something extra it might be associated with your mystery-switch.

Fused switches are usually located right next to the appliance they control e.g. next to oven, next to immersion heater.

The fused switch seems to be in an awkward location for anything outside the room (e.g. outside floodlights, immersion heater on other side of wall).

Perhaps a voltage or metal detector will detect that the switched live wire leads to a spot on the wall where an electric fire has been removed?