If you book a UK train ticket online, for most routes, you're given a 8 character code. You then head to a UK railway station, go to the ticket machine, tell it you want to collect a ticket, give it a bank/credit card, then enter the reference, then the tickets are printed. Loco2 have a handy pictoral guide to the process
The bit that I'm confused about is why it asks for a card during the collection process?
If you put in the card you'd booked with, and it then printed out your tickets without asking for the booking reference, I could see the reason. If it was a security thing and would only let you print if you entered the same card you booked with, I could see the reasoning. However, it doesn't seem to matter what card you put in, it will still prompt you for a booking reference, and then still print your tickets once you've entered the correct booking reference. Doesn't seem to matter if it's the same card you booked with, one of your other credit/debit cards, someone else's credit/debit card, or even just a supermarket loyalty card with a magnetic strip in the same place as a bank card. Behaviour is all the same, prompt for a card, apparently ignore it, then ask for the booking reference.
Edit OK, so only certain Supermarket loyalty cards work identically to bank/credit cards, accepting the card then prompting for the reference, many other loyalty type cards are actually rejected, but by no means all!
So, what's the point in asking for a card at all? If it doesn't matter what card you use, and if a supermarket loyalty card is good enough (which it was earlier today when I collected some tickets!), why do they even bother requiring you to put in a card before you can enter the booking reference to collect your tickets?