iphones are sold with contracts only, and while they can be unlocked using third party hacking of the software, everytime the opperating system is updated it tends to mess up the hack, so you have to start all over again looking for an updated hack.
You will also lose out on the visual voicemail which comes via the contract and of course the unlimited data plan.
On the plus side, they are a lot cheaper in the US and will work fine over in the UK using your existing SIM card and the hacked software.
To be honest i would rather pay the extra and make sure your covered through Apple in the UK for one of the official UK iphones despite the price and the 18 month contract..