Does anyone know if it's possible to hire, for a day or three, a good-quality carbon/ti road bike, in south London or Surrey?
Head for the hills dorking?!
I've not used them but British Bike Hire might have what you want. http://www.britishbikehire.co.uk/RoadBikeHire
Further afield for carbon-fibre is Totally Bikes (Newbury, Berkshire): http://www.totally-bikes.com.
They do collection and delivery to the London area. Bikes can be picked up at a triathlon shop in the Windsor area if necessary. Deposit required, but you can even borrow an ex-Ben Swift Pinarello if you want to be really flash.
I recommended you for On Your Bike which is biggest retailer of road bike and London Bikes.
I know Pilgrim Bikes at Westhumble Station (bottom of Box) have road bikes to hire, but IIRC they're more basic than you're after. May be worth a call though.