Originally Posted by pseudoswede

Air Transat is not Air Canada.

If things go well, then traveling with Air Transat (IATA code TS) will be fine. If things go sideways, then you could be in a situation where you may not be rebooked to another flight for a few days (they only fly LGW-YYZ on Tu/Th/Sa/Su).

Flying with BA gives you many more possible options to fly if LHR-YYZ is severely delayed or cancelled. They will involve connections (and you'd have to make sure that you're allowed to transit through the connecting airport), but at least you'd be on your way.