iPhone 2G is ancient, approaching 4 years old. Even if you could get one it simply wouldn't be worth paying for one on a contract.
You can get one for about £120 on ebay.
Most contracts will be for the iPhone 4 or iPhone 3GS.
A contract isn't the cheapest way to get one either, I bought my iPhone 4 outright and I'm on a PAYG operator called GiffGaff. I reckon I've saved quite a bit over a £35 iPhone contract. But obviously if you have no savings that's not an option.
Edited by gilesjuk (Mon 21-Feb-11 21:54:27)