There are plenty of gateway cities that have BA transatlantic service but not AA.
1. Open an Alaska account, book BA transatlantic miles to AS, everything else to AA. Still 25% for discount Y, but better than a poke in the eye... or,
2. Go with IB. Besides, MAD is a more pleasant place to connect than LHR. Use the AA codeshare on IB6242 and get full AA credit.