Also what rating would be suitable for running a spur to a shed, would
a 16A MCB/RCBO be suitable?

Not really, if you going to wire it as a ring circuit you can use a 32A mcb and
separate 6A for lights.
Q Have you thought about earthing, as discussed in this group power to
outbuiildings should be RCD protected at the outbuilding itself and use a TT
earthing system ie, earth rod . And if your outbuilding cable run from your
distribution board is getting near to 30M use 4mm sq to allow for v drop