I am the parent of a high school student going to study in the UK this summer. We have an unlocked quad band phone and I need to find a pre-paid sim card for the UK prior to him leaving the states. I have seen them on Telestial's site and from what I can read here, they sound acceptable. But I also wondered if you can pre-purchase the "top up" vouchers in advance. The students may not have the freedom to nip out and find a kiosk and I do not anticipate giving him a credit card.

Any suggestions for pre-paid sim cards but also any way to purchase "top up" vouchers ahead of time?

Thanks!