Do NOT solder T&E.

The only soldering an electrician will do is to add bonding in an innaccessible place. This is on single-core wire, so melting of the sheath is not as much of an issue, and it will be soldering/brazing direct to a copper pipe.

Why not extend the ring properly?

If you must add a spur, you are allowed under regulations to take it from the socket.

FAILING THAT - BUY A JUNCTION BOX - IT'S WHAT THEY'RE FOR.

About £2.50 for a 30Amp box.