If dealing with the local authority when it comes to tenants, they demand a Seperate power source for a shower fan, I had to do it in 2018.There's nothing in the regs (linked above) that states that, they just state 'Adequate ventilation shall be provided for the removal of water vapour from every kitchen and bathroom. '
Separately, under building regs, TGD F deals with ventilation requirements. Where there is a window with an opening of sufficient size to provide purge ventilation, that meets the requirements. For situations where mechanical extract ventilation is required, connection to the light switch is fine, but in some instances a timed over-run of 15 minutes is required.
Stupid admittedly but there you are, in a private tenant situation, the regs are sometimes less strict