you need to bond every metal service as it enters the bathroom (or just outside, e.g. in an adjacent airing cupboard). Once you have done that, they can't pick up a voltage inside the bathroom, because you have already bonded them to the CPC of every circuit that enters the bathroom (lights, shaver socket, wall heater, immersion heater, shower pump, etc).
So, once you have bonded e.g. the cold water pipe where it enters the bathroom there is no need to bond it again and again at the basin tap, bath tap, bidet tap, shower mixer and WC cistern.
Once you have done that, all the pipes and taps and radiators will be at the same potential inside the bathroom (even in the event of an electrical fault). So you have made the bathroom into its own equipotential zone regardless of anything that might happen inside the bathroom, or outside.
