Originally Posted by
Stanbury
Actually, that's not been my experience when transferring to WestJet in YYZ T3; checked bags are transferred directly to the domestic WestJet connection. However, my experience has been limited to Air France to WestJet, but I do not see why BA should be different. This is not done for transfers to AC, when it indeed works as you describe (as AC use a different terminal). And just FYI, you will have to go through security again at YYZ T3 (up one level).
Canada's International to Domestic transfer program only includes certain airlines. As I understand it, Air France to WestJet connections are eligible, which is why your bags were transferred automatically. From all available evidence British Airways does not yet participate, so the old process (reclaim bags in YYZ, drop them back on the connections belt) would take place for BA-to-WS connections.