From my understanding you need to cross bond all metal pipework in the bathroom together, unless it's plastic and just a short piece of copper sticking out the wall, then don't bother.
You need to use 4mm cable and also you need to run that 4mm also to any electrical circuits in the bathroom, normally just a lighting circuit.
You can use a split load board with two rcd's instead of rcbos, i think you may need to check out about smoke alarms if they can be on on rcd, some boards have facility for non rcd as well as two rcds.
I will give the NIC a call monday to find that out for myself.