Up in my loft I need to add a power socket which will be used to power one
tv amplifier.

There is the lighting circuit up there, and conveniently near to where I
need the socket, there is a brown juntion box which just has 1 twin+earth
in, and one the same out. Perhaps someone used this to join the cables?

Anyway. Can I just connect a length of twin+earth into here, then connect
the other end into a 5A single socket.

I'll have to cut the flex from the TV amp and fit a 5A fuse with the rounded
pins to fit the socket.

Does this sound OK?

Ideally i'd like a double socket up there because for a while whilst i'm
installing the aerial stuff i'd maybe like a spare "normal pin" socket
available to charge a small power tool, but these perhaps could be
misinterpreted as a "proper" socket in the future and someone might overload
the lighting circuit?

I'm also understanding that this wouldnt be notifiable under part P? I do
have some old cable length up there I could use.......but then again the 5A
socket I buy will probably have a date on it somewhere.......

Any comments would be gratefully received.