Look here:
http://www.simpletalk.co.uk/bt.htm
You buy a SIM card for £60 and it includes line rental *FOR LIFE*. You just stick the card in your existing phone and away u go.
Failing that they do a Phone *AND* a Lifetime SIM for £139!
You can choose a tariff that includes 100 off-peak minutes and 20 text messages FREE per month.
If you exceed the free quota it's 2p per minute weekends, 5p evenings and 35p weekdays.
Other tariffs are available with up to 200 minutes free per month or another with 50 texts per month.
Just think, for a £60 outlay you could have FREE mobile calls for life, provided you stick to less than 200 minutes/month.
Phone/email them and ask about their Cellnet "Pay upfront for life" SIM's. You want to talk to someone called "Taz".
(BTW, I have nothing to do with "Simpletalk" I'm just a happy customer.)