found a similar thread on another forum :
"You could do a fused spur (13amp) off the ring, use an 2.5mm SWA to feed an RCD double socket in the shed. You would have to do another fused spur (3 amp)from that socket to feed a light."
"spuring off the socket circuit is allowed but don't forget only 1 socket per spur unless fused.
You would need to use armoured for burrying, and also would need rcd protection and bonding.
Your cheapest option would be to install an RCD spur in the house, come out of this with an armoured into the shed for a couple of sockets with a switched fused spur to switch the lights. This means your maximum load will be 13A."
"The problem you've got is leaving enough capacity in the ring for everything else it serves and not throwing the ring out of balance (it's easier to overload the shorter leg of the ring if a large point load is away from dead-centre (very easy if it's less than one third of the way around)."