I wish to operate LV halogen lighting via mains lighting circuit/transformer and a shower fan via the same mains lighting circuit from the same light switch using box for all associated mains cabling including transformer supply. Is this permissable and if so, practical?
It is normal to run fan and light off same supply the only point is fans do go wrong from time to time and an isolator is a good idea there are special three pole versions specially designed for the job.
230 volt is low voltage under 50 volt is extra low voltage only problem in using extra low voltage is connecting items as often the terminal block is not big enough.
So some form of junction box will be required and screw type need to be accessible Hagar make some specials that don't need to be accessable.
Also of course in bathroom it comes under part p so will need to be correct for local authority inspection. This will include now RCD protection.
In the main it is cheaper to get a registered electrician to do the work than pay local authority their fees.