You carry out supplementary equipotential bonding in a bathroom which is different to earth bonding, it is local to the bathroom and does not return to the CU. (unless the CU is in the bathroom )
Also see: http://www.diynot.com/forums/viewtopic.php?t=57896, the post by BAS has some useful links.
The cold water main passes through my bathroom and the copper pipe is connected (at the bottom) with a 10mm main earth to the MET; so I suppose, by chance, it is earth-bonded to the CU... but regardless of this, the equipotential bonding goes to the lighting circuit (there are no other circuits) and the various hot, cold and c/h pipes.
The point of equipotential bonding is that it carries on working inside the bathroom, even if someone hacked about at the cables between the bathroom and the CU.
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