Not that I know of, unfortunately. The way a ba.com PayPal transaction is processed, is that a onetime 'credit card' number is generated and used as the form of payment. This card number cannot be accessed or used again.
Sounds like ba.com falls over after asking you to login to PayPal, and reverts back to asking for the 'credit card' used for the purchase.
You'll need to call up I'm afraid.