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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-
All good info, cheers. I'm pretty sure I'm meticulous enough to fit the stuff right, but will probably get someone in to wire it up, and drill the extra holes in exterior walls that the vent unit will require. I'm going to consult back here when I come to that as there's few options regarding products and layout that I need to chew over.
Polygon
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.