This is how a lot of payments are conducted in Germany. It's easy and secure for the customer as he can reverse fraudulent charges.

What makes this different than providing websites with your credit card details?

liability in the UK is different: credit card transactions are heavily protected (you're guaranteed to get your money back), whereas direct bank transfers are vastly less protected (with the exception of direct debits).

can you even have a one-off direct debit? I didn't think the system was really designed for that.

This is a common misconception about direct debits. Direct debits can be used to charge people varying amounts at varying frequencies (including one-offs).

In this situation it is a direct debit so has very strong protection in place.

Yes a DD can be used to make a single payment, it can make a repeat payment of a varying amount or a fixed amount. it is quite a flexible system.

Presumably you'll set the direct debit up on the GoCardless service, and then authorise each payment through them. So just like a third-party credit card processor, the merchant never sees your details, just that the payment has been authorised.

This is exactly how our service works. We never share your banking information with anyone.