Remember this is a shower-fan/light with a SELV halogen light in the vent over the shower and an in-line fan in the loft venting to the eves. Hence the isolation is by the fan in the loft.
Yes it goes through the FCU first and then to the switch, the shower-fan/light unit instructions specified this with a 3 A fuse. The switched FCU isolates the entire shower-fan/light and switch circuit. Yes a 3 pole isolator after the switch would give the same isolation but wouldn't fuse down as per the spec.
It's switched separately from the other lights so you can have the fan and shower light on when you are in the shower(with 10 min over run) but not when you pop in for a pee and only need the other lights.
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