Nothing at all wrong with a plate switch in a domestic bathroom. There is, and never has been, any regulation preventing it. The original regulation was that it shouldn't be placed within reach of a person using a shower or bath, and this was relatively recently changed to the 0.6m regulation mentioned above (which in practice, is much closer than permitted by the original reg.)
I have never fitted a cord switch in any bathroom or shower where the regs could be satisfied by a plate switch, and have NEVER had a single instance of a customer complaining of even the mildest electric shock from a switch.
I have, however, had several cases of other electricians, like in the OPs case, complaining that it's not allowed, including the QS of a large local electrical contractors. This happened on the single occasion that I've had to use a third party to sign of a new build to the local authority, after a state of war had existed between the builder of the property and the LA.