Are you already on contract? It is much easier to get a good deal if you already are, and are coming to the end. As an example, I managed to get for £20 pm 300 mins, ultd texts and unlts internet. Plus a very expensive phone. My brother also managed to get, as we're both on vodafone, for £12.50 pm 100 mins, ultd texts, and ultd internet and a free Google Nexus One.
BTW,even though they are 2 year contracts, you can often exchange the phone you get even when you're well in. I've had a few - in this order - Nokia N97 mini, Nokia N900, and now a Sony Xperia X10, which is definitely the best of them. But iPhone for £20 pm? Nah, its not going to happen - though you can get iPhone on £25 pm.