It seems odd to me that the GAs in LHR don't know how to process a "ticketing" function like an upgrade that requires tax to be paid. This can't be a once in a blue moon phenomenon. I'm surprised that the gate supervisors don't know how to do it or that the gate agents can't just call some support person and have them do it remotely. It also shouldn't matter that this isn't a "typical" European function... it is American Airlines after all, and the point of having a brand is to standardize the product and experience no matter where you are.