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You would need a registered and qualified electrician (registered with a scheme) to install a new circuit. If the cable is 2.5 t&e then it would be fine for a socket, above that is also fine. They can establish the correct rating for the breaker by testing the cable and testing the installation to confirm. I guess it could be smaller twin & earth, i.e. a lighting cable which would generally be too small for a 13amp socket.
Depending on the age of your installation they may have to upgrade other aspects, very common to need a 10mm earth to the water/gas supply if your installation is over 15/20 years old.
Airhead-
Thanks, I think it's 2.5. As happens it might be a bit moot as the reason to do it was too get everything in place to house the ONT for a fibre connection up there and avoid having the fibre line running down the front of the house and ONT in my lounge. Looking in to it a bit more I'm not sure that's possible as there's still the fibre splice box mounted on the outside wall and I doubt BT want to put that up high.
Polygon
It's actual a new circuit. There's a cable coming in to my loft, not sure where it comes from or if it's entirely suitable, but my best guess would be a circuit for an electric shower that was never installed.
If it does lead back to the CU then my plan was to use it to get power in to the loft, no idea if that's legit, hopefully a qualified electrician can tell me.