If the flight was a direct connection from a transatlantic e.g London/Toronto/Winnipeg with Toronto/Winnipeg cancelled then EU rules will apply. If it's the other way e.g. Winnipeg/Toronto/London with Winnipeg/Toronto cancelled then Canadian rules apply. If you stopped over at the connection point e.g. London/Toronto day 1 and Toronto/Winnipeg day 2 or later and the Toronto/Winnipeg is cancelled then Canadian rules apply. If you happened to simply buy an Air Canada flight domestically and from an EU agent without a transatlantic on the same ticket Canadian rules apply. Also remember that the UK now has its' own rule and while they are similar to EU the complaint route is not the same.
This means advice needs you to give more data about precisely what happened.