Pay as you go phones start from about 40 pounds; look at Argos or woolworths for the easiest ones, check which tarrif you are going to be cheapest with (if you have a BT line then O2 is probably best). Buy the cheapest handset you can get; Nokia is most likely for other people to have a spare charger about if you lose/forget yours, Siemens handsets tend to cost a little more but are hardier (esp in the screen department) and sometimes have atune composer- yay!

If like me and dr_bob you're a relatively light user, PAYG is by far the cheapest and best (I used to spend 15 A month on contract as a frequent user; now it's more like 15 a quarter, and dr_bob is even less. It's very very easy to top up PAYG; simply go to your own bank's ATMs and you can top up at the cashpoint (including LINK machines if you have a LINK compatible bank card). You can also top up by debit/credit card by phone (bit of a faff) or get an electronic topup card which you hand in at participating supermarkets/convenience shops/post offices and then they add topups on for you instantly; which depends on your provider.

Good luck and if you have a composer on your phone and want a customised ringtone let me know!