There is electricity coming from a 13a socket inside the house which is going to the light bulb inside the outbuilding.
Firstly, Supplementary Bonding in the location -
If the light has no metal (exposed-conductive-)parts then there is nothing to bond.
Even if it has, supplementary bonding is not required in WCs but you could bond pipe to e-c-p.
Secondly, Main Bonding -
Main water pipe coming out of the ground is not bonded.
Do you mean in the WC or in the house?
If you mean in the house, it should be main bonded to the Main Earthing Terminal.
If you mean in the WC and it is a continuous metal pipe from the house where it
is Main Bonded then no need - not much you can do about it anyway.
Ideally, a bit of plastic pipe in the run to separate it from the house metal pipe would be best but still doesn't need anything in the WC.