Quite - and I strongly suspected if you asked the manufacturers of a CU whether it was suitable for installation in a humid environment, they would probably say no.Hang on ! Note the "due consideration of evironmental conditions"
Furthermore, as you will probably recall, there has been previous debate about the assertion/interpretation made in the answer to which you link. That answer suggests that the inside of a cupboard which is located within Zone 2 of a bathroom does not count as being in Zone 2. However, if one assumes that the primary reason for the Zone 2 restrictions relates to risks of users of the room touching electrical equipment, then common sense would suggest that the above-mentioned assertion would only really be appropriate if there were no possibility of the cupboard door being open whilst the room was being used. That could really only be (almost) achieved with a locked door - and the potential issues regarding that have already been mentioned. Quite apart from the CU, the DNO might to be happy with their equipment being in a locked cupboard.
If (as seems to be implied) there is another room on the other side of the wall, might it not be possible to have a totally 'closed and sealed 'cupboard' in the bathroom, with access through the wall from the adjacent room.
Kind Regards, John
