I have a feeling they've all changed recently, as they all seem to advertise that 'all our packs/goodybags/whatever-they-call-them last for 30 days'. Vodafone offer an 'only pay £1 on the days you use your phone', which should work out pretty cheap if you never use it, but even a monthly test is going to work out at £12 a year.
I've been casually looking for some time now, for a 'Stick a tenner on it, and it'll last forever until you use it up' deal, even if it's actually something like 50p per text, but still not found one.