We found it not terribly expensive to purchase a prepaid cell upon arrival in UK. Very handy for phoning ahead and confirming reservations, placing new reservations for tours/boat trips etc., and (on trips with multiple parties) keeping track of who was where!

First time, we borrowed a UK prepaid from friends who had done just that. If you do purchase one, bring it home, you'll likely be able to resell it on this board to the next traveler. We found it easy to add minutes at any fuel station convenience store, etc.