It would be a lot cheaper to buy an H class fare and upgrade it from YVR-YYZ-YOW-YUL-LHR and return which would earn your about 20000 miles if you are eligible for targetted extra status miles and IF you could book it on destina in H class (I tried without success for a similar trip and it gave me errors about too many cities on one day). The only reason I say destina is for the 1300 status miles. It is the CHEAP way to do it though. If destina doesn't pan out on line, you could phone them for assistance. They will waive the help charges if it can't be booked on line. What is not clear is whether they will give you the status miles.

Upgrade with SE certificates each way all the way. Lots of options for flights. YUL-LHR is usually not too bad compared with YYZ-LHR. At this time of year, seems to be lots wide open. I know you don't really want to go back to UK but I'd rather save the 3700 bucks for next year's travels if you aren't at least going to stay for a day to enjoy Hong Kong.