rather an odd one
A is doing some marketing with B and setting up a website. B has a business involving payment methods for tourists visiting the UK, a bit like having a prepaid Credit card
So far so simple. A is providing a VATable service to B. We don’t act for B
However, B is charging commission and when more than a certain amount is spent, the customer is refunded part of the commission by A. A has the details because A runs the signup website
A is not doing anything for the tourists. If anything, A is making a third party supply on the instruction of B.
I tend to think, therefore, that A’s VAT liability is purely on what it charges A and it does not need to worry about the tourists as it is not supplying anything