I believe that AC will interline with most other airlines (that issue is different than the issue of an joint electronic ticket for an AC/other airline itinerary - such airlines, incl. BA, are listed on the AC.com/ca web site). Therefore, as mentioned by a previous poster, if I want to go the LHR from YOW on BA, and I want to fly all the way, my options (assuming I do not want to go on Porter, and worry about a airport change in Toronto) include AC and Westjet from YOW to YYZ (or perhaps YOW to YUL), and then from YYZ (or YUL) to LHR. In this scenario, one would have to switch terminal buildings : T1 to T3 - and it would be advantageous to transfer between the 2 buildings airside, if it was possible. Coming back, one would have to go through Canadian customs and immigration, to the landside portion of T3.