Originally Posted by
Happy
AA does not fly YYZ-LHR. I doubt AA would endorse OP's tickets to just ANY carrier from YYZ to LHR.
OP has to rely on BA's inventory. BA is normally very accommodating if there are seats available. However BA cannot simply accommodate OP - the action has to come from AA.
The IATA convention calls for the carrier delivering travelers late to the connecting point to be responsible for onward travel. So, the proper contact point is AA. And yes, AA would be expected to endorse tickets to any carrier with which it has an interline agreement, no change fees, irrespective of alliances, award inventory, or any of that nonsense. BA is a logical choice, but Air Canada would work just fine.