What Welchyboy1 said
The Quooker is only 1.6 or 2.9 (if you get the big one) kW and the instructions say to use a 13A socket.
If you can't just change to a switch with a socket, then just add a single socket as a branch of the radial - it does NOT need to be 10mm cable. Assuming it's only short, then it actually only needs to be 1.5mm to feed a single socket - as the plug is fused, the cable feeding the socket is protected by this fuse and the regs allow for the cable to be smaller than would otherwise be needed for the upstream MCB rating. The biggest problem will be terminating it alongside a 10mm cable into the terminals in the JB.