sorry I missed your reply,
Here goes as I understand it, part f of building regs that ventilation is required in kitchens, utility, and bathrooms. Not WCs.
This applies to rooms where the windows are openable and are located on external walls.
In rooms without openable windows the same applies but fans must have a 15 minute overrun.
When rooms have electric lighting it is usual that these are used to power the fans too. This may not be convenient for your electrician to do but I wouldn't totally agree with putting the isolators outside the rooms, could depend on the zones within the bathroom but in the other rooms that is not an issue unless a bath or shower are installed in these rooms.
But it's difficult to see the logic behind your electricians thinking, it could well be the senisible option.
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