You can use AA in combination with BA on a European award, but just not on the international sector. I have booked for the spring - AA)ORD-CDG(stop)
BA)CDG-LHR-AMS(stop/destination)
BA)AMS-LHR
AA)LHR-DFW (stop)
AA)DFW-ORD
All for a 40k award(1 international stop, 1 international destination, 1 domestic stop).
So, in my opinion AA gives you the most flexibility