It is really up to the agent to enforce the requirement. Some will ask you to use the sizers, others will not even take a look. Your bag is 4" wider, which means that it will protrude out of the sizer if you were forced to do the test. If you don't mind having your bag checked airside, just make sure they tag it correctly to your final destination.

On a side note - since there's no interlining in this sector, AA's baggage policy will prevail, regardless of the fact that BA issued the ticket.