I see that no one has answered your actual question so far.

No, you cannot use your miles on a flight that says AA but is actually a British airline (i.e. a "codeshare"). If you asked the agent how to do this, he/she undoubtedly said it was impossible.

However, you CAN use your miles on a flight that says AA and is actually AA; and you can also use your miles on a flight that says BA and is actually BA. If you insist on flying BA, you will have to pay high fees, there is no way around it.

Your miles are valid not just on AA, but also on a number of partner airlines. But you must always book through aa.com or by calling AA.