I've just found out about a really good Pay As You Go deal. Asda Mobile have an excellent tariff. Find a suitable (unlocked) handset  and get an Asda Mobile SIM in it. It's 16p a minute to ANY British mobile network or standard British landline number (beginning with 01 or 02 but NOT 08). After 3 minutes have been clocked up in any one day, this goes down to 8p per minute during that same day. (YES! that's VERY CHEAP cross-network calls!) Texts are 10p each but after 3 texts have been sent in one day, further texts in that same day are only 5p. Top-Ups via credit or debit card are awarded with an extra 10% of airtime - so top your phone up with £10 from your card and £11 will go on your phone. This extra 10% top-up bonus also applies to voucher and swipe card top-ups if they're done in an Asda store. If you make a service call to their Customer Services line to register your mobile number, it costs 25p but they'll give you a free £1 top-up a month later!

They're a service provider that use the Vodafone network (although they're a completely separate company, of course) and that means it's great coverage and you can pick up a signal just about anywhere - some networks like T-Mobile and ORANGE have patchy coverage in some areas or or signal blackspots/weak coverage but Vodafone is generally agreed to be a good signal over more areas than any other network.

Check out their website: http://www.asda-mobile.com/