These days is does, but probably at the time of installation (16th edition) then supplementary bonding was enough for a bathroom installation.Ok well I'm not familiar with bs7671 don't work here ,so can be shot down
The Best Practice Guide #4 would have C3 for this case (no RCD but bonding good) or C2 if no supplementaty bonding when inspecting things.
So long as the CPC is good then ADS is ensured.How long until the heater tank develops a fault
Of course, even if the regs did not say so you would still want an RCD for other reasons. Like protecting against the results of cable damage, or to reduce the risk of a double fault being fatal (so #1 loss of CPC and then #2 insulation failure).