To try to sort a few things out here:
An iPhone 4S costs between about £500 and £700 if you buy it without entering into a contract. If anyone is going to let you have one cheaper (or even free) they'll need to recoup their loss through what they charge you for using it. So they'll want you to enter into a lengthy contract. Unless you pay a really hefty upfront fee and/or extremely high monthly fees, you won't find any 6 months contracts (and you're unlikely to find any 12 month ones). To get a fairly low monthly rate you'll need to sign up for a 24 month contract.
A contract phone is likely to cost anywhere between nothing and about £170. The more you pay for the phone, the lower your monthly costs will be.
The search facility on the Car Phone Warehouse website is a good way to see what you get for your money with different contract deals:
http://www.carphonewa...mobiles/iPhone-finder
(Car Phone Warehouse might not be everyone's first choice as a phone provider, but they deserve full marks for the clarity of their website).
Chris