You should not be doing this work - and not because of Part P, but because you just don't understand anything about what you need to do.
well i cant find any people wanting to put a su8nbed in there shed on a dedicated circuit so my question has not been done to death at all, if you cant be bothered to give advice then why run a forum or be a member of one,
It doesn't matter that it's a sunbed - it could be a hot tub, a lathe, a kiln, a welder, an A/C unit, a milking machine, a socket for a lawnmower - anything at all. All that matters is that it uses electricity and it needs a certain amount of it in order to work. The fact that you have discounted all the information on getting electricity out to a shed because the intended use wasn't a sunbed shows that you understand nothing about electrical circuits.
its fine getting the electrics in for the sunbed what about the light & socket, can i run 4 core swa use L1 for the sunbed the use L2 for the socket which is needed then come from that with a switch fused spure for the light replacing fuse with 3 amp one, ?
You are supplying electricity to a shed. Once the supply is there you can create whatever circuits you need. You said yourself that the feed for the shed is a single MCB at the house end - how on earth can you therefore create multiple circuits from there?
In all the advice you've read here, have you not once seen that which describes a cable supplying a CU in the outbuilding with individual circuits coming from that?
all i need to know is,
1, can i run the power for the shed off the spare breaker,
Yes.
Depends on the load in the shed.
shall i fit an isolater sunbed end to isolate power from sunbed when needed,
Yes.
Depends on the load of the sunbed.
will 6mm swa be enough, thats all i want to know
Depends on the load in the shed, and the length of the run, but you'll have seen all the info on that when you researched this topic here, as it's been done to death.