Tesco Mobile have a feature named Ping Me A Pound which lets one Tesco Mobile customer grab a pound credit from another Tesco Mobile customer (with that customer's agreement!).
The instructions on Tesco Mobile's web site aren't quite right though (at the time of writing, that is).
The Funder (the customer who's making the pound available) needs to logon to her Tesco Mobile account on the web, go to My Cards and Services option and add a Child number in the Ping Me A Pound section. One can have up to 10 child numbers active.
The Requestor (the customer with the Child number) doesn't need to set anything up. Once the Funder has added the child number to her account, the Requestor just has to dial 2703 (free call) to receive a pound credit from the Funder. This can be repeated (only once per day, mind you) until the Funder removes the child number from her account, or no longer has a pound credit to transfer.
Having set up the child numbers, the Funder receives no notification when a pound has been transfered to a child. One has to regularly check one's account balance.
If 10 pounds is transferred to a child, and the child is on
the standard tariff, the child receives the additional
20 pounds free credit for topping up by 10 pounds!
Perhaps if the child then transferred the 10 pounds to someone else (or even the original Funder) that person would receive the 20 pounds free credit too...
Having said all this, my favourite mobile operator is actually Giffgaff