Busby,
I think you'll have difficulty finding what your after with VoIP at the moment. I'm assuming you after something similar to a mobile top up card from O2 or Orange etc...?
The only providers who i think will offer what your after are the large chain supermarkets and electronics companies. I.e. Dixons, as you've already experimented with, or my only other suggestion at this time - Tesco.
I know you purchase a voucher type affair to begin with, but I’m not sure if you can the purchase another one to (top up with). Obviously your only going to find these vouchers in Tesco, not over the counter of every shop in the UK.
Matt
