On topic, and something that I thought of..
My experience with using these 'debit' cards (through watching EmmyLou!) is that while a lot of sites WILL accept them, there are quite a few that need full credit cards to make purchases.
The reason for this, as explained by Tesco Onlie helpdesk is quite logical really.
When we asked them why they would not accept Emmy's card, they explained that as these sort of cards were freely available to people under 18 (Emmy has had hers since she was 12!) they could not take them because if they did, there would be nothing stopping kids buying booze, 18-rated films, or even (Heaven forbid!!1!!!) 18 rated games like GTA4
Therefore, the reasoning is sound, even though it is a PITA.
It doesnt work in all cases though, for example, HMV accept them for online purchases, and NO age checks! (so buy your violent games and soft pr0n from HMV!
)