roshan works but can be a bit tempremental. check the cost before you go! the text cost on o2 for me was about 40p (not too bad value for keeping in touch) but the voice call cost was massive. if you take a phone you can buy a sim card and go pay as you go.
if you are near or in a large town then you should get (patchy) reception.
Sounds like a good back up to Paradigm. I have Virgin PAYG and pay direct debit, according to Virgin incoming calls at 50p minute and texts are 25p so bearable, similar to roaming in Europe and cheaper than in the good ol uSA at 80p incoming voice.
A Roshan SIM would be top looks like a call to UK would cost about 25p with incoming calls free. Pity EFI doesn't sell them, presumably cos the grown ups will throw their teddy bears in the corner.