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Decided then! I'll get an electrician in to sort it out. That's mostly well over my head and all above my current skill level.
Polygon-
It's probably for the best based on the kind of question you're asking. You could do it and survive following common sense guidelines but it wouldn't come close to the level of confidence that fully testing the circuit once installed can give you.
When an electrician looks at the whole installation he can establish the course of least resistance to provide a safe connection. If you have spare ways and a short run from the distribution unit then it would be easy enough to provide an rcbo if your distribution unit has no rcd protection already. RCBO's are more expensive than RCD's but they can disconnect the supply without affecting other circuits.
Electricians are obliged to check that there is nothing causing immediate danger in your installation but they do not have to bring any circuits that they are not working on up to current regulations. You should also get some paperwork showing that the work has been tested and the results, if they are self-certifying (which is what you want) they will also pass paperwork to the building inspector.
You can always install the hardware without wiring it up, that saves a bit of time and money. If you like things done to perfection it's probably the best way, I've seen some hideous fixing jobs with towel rails.
Airhead
If you are adding them to the bathroom you should probably get an electrician to do it. The bathroom is one of the few remaining "Special" locations where electrical work is notifiable. You can replace existing accessories although you are still obliged to make sure the work meets standards set out in BS7671.
If you want to carry out the work yourself you can apply to the local council and pay to have the work inspected by them, it will cost between £150-£300 depending on your borough.
The extractor fan should be on a double pole switch.
You need to know several values unique to your installation to establish the rating of the fuse and size of cable for the towel rail, these values can be determined by testing. For example your earth fault loop impedance and prospective fault current and the Zs value (resistance) at the towel rail. As it is a room where you are likely to be wet and naked the fuse rating is important as well as the RCD protection which rive mentioned.