Hi Steve,
This is the response I got from TfL about registering international bank cards:
Due to not all non-UK bankcards complying with the required standards around security, particularly in relation to the new Strong Customer Authentication legislation which came in earlier this year, overseas customers may not be able to use their non-UK bank cards online. Once non-UK bankcards begin to adopt these new security measures, we hope to be able to allow these customers to register their cards, and therefore be able to check their journey history and apply for refunds more easily.