three's mix and match seems like a good option to consider, bearing in mind we don't know how many minutes and texts you use per month. 500 minutes or text (or combination) for £18 and then pick a handset to go with it.
The latest handsets on three will attracted an additional monthly payment (upto £9 per month) however there are some helf decent handsets on there. If you are looking at nokia, then consider the 6500 slide, which is f.o.c. or the nokia 6220, which attracts an extra £5 charge on top (£23 per month total)
For some reason, the network operators want us all on 18 month contracts. Yes, there are 12 month contracts out there, however you will be charged more for the priviledge, and in some cases, will actually get less for your money.
You should also consider the sim only options, which allows you to cancel the contract with a months notice (rolling 1 month contract. This option gives you control on how long you want the contract - only downside is you have to have your own handset, or purchase a PAYG one and use the contract sim. The sim only option starts off at something like £15 for 300 minutes of texts (i think virgin are doing something - i remember a leaflet coming through the door a few days ago)
Give us more info and we can give more answers