Why not from the lighting circuit? Isn't that the way to wire them if
you want them to switch on automatically with the light switch (eg as
in the case of an internal bathroom/toilet)?

Thats the way we wire them.
From the bathroom fitting we take a 3core and earth (6243Y) to a 3 pole fan
isolator (outside of the bathroom) then another 3core and earth to the fan
itself. The 3 core carries a permanent live, a switched live and a neutral.

--
Gary
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