Sorry NEMMIE, I don't think that this is possible with BA's booking system, although if you book the flights over the counter at a travel agent it should not be a problem.
The other option is to pay money into your AMEX account, giving you a credit balance -- you should then be able to spend the required amount (although it would probably be best to call AMEX in advance to clear this, in case a transaction limit has been set on you account).
Let us know how you get on.