Depends on whether you want a contract deal, or PAYG, and what you want from your phone.
Cheap and cheerful, could try Argos or visit an Asda/ Asda Living (they have phones for most networks and sell unlocked PAYG phones, so you can choose a SIM later). Argos and nearly all online phone stores force you to pay for a top-up (which means a cheap emergency phone for your network costs a tenner extra, sometimes more, than the price first spotted... CPW do cheap PAYG too, well, cheap when the cost is from say 5 quid to 150 quid)
For PAYG, Asda (using Vodafone network) is one of the cheapest at 8p/min for voice calls to mobiles or landline (01/02/03) and 4p/text
If you are a massive texter then CPW's Talk Mobile deals are perhaps better as you can get up to 3000 texts a month for about 20 quid, I think.
So much depends on your use of phone, and whether camera, music, radio, etc need to be in it.