For a mains bathroom fan in zone 2 / 3, where should the DP isolator
be located?

I have a redundant immersion heater supply running to my loft, via a
switched spur in the downstairs kitchen, If this spur is replaced with
a 3A fused switched FCU, would this location be acceptable? Fan will
be a humidistat type, hence no requirement to connect onto lighting,
remote pull cord etc.

thanks
DD