Thanks @Luz - there is hundreds (literally) of pages on this Vodafone UK issue already.

To summarise for new people, the following can't/won't happen:
"has anyone actually been charged and therefore been able to envoke the T&C's clause whereby the user has a right to terminate their contract"

The way the contract is written they can alter the FUP without it being a breach (and therefore giving you an automatic ticket for termination/suing). I specifically say this to those that signed up on the 'original' (i.e. 2009 and before) contracts. The data limit is subject to a FUP which they can update and their discretion. They aren't stupid (unfortunately).

That said, don't take the piss and you won't have any trouble. You don't need to be doing over 2GB a month on 3G, and on Voda it's got so slow anyway in most places it's hardly worth it.