Depending on your FT nerdiness, you could book LHR-YVR v/v and the buy the YVR-YYZ v/v on AC separately. You may find that that is not much more expensive (perhaps cheaper) and scores you some extra *A miles you can deposit somewhere.

I'm not sure about the CX recommendation. Yes, the intl CX product is better than AC transcon but are you planning to buy a F or C ticket or go Y? If it's Y, the Y bother (and put up with the US hassle?)