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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-
Those boxes are a pain. I had Community Fibre put one on the first floor because I already had some internal internet infrastructure there. I regretted it within a week but the deed is done. Painted it to match the walls and I don't see it/look at it anymore.
They've recently done the half the street and there are barely 2 installations the same.
Airhead
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.