FRA is always a tough route to upgrade, and I believe that you need a higher fare class even to be eligible for a *A upgrade. There are multiple flights to LHR from YYZ, as well as the YUL and YOW flights, and LHR is usually fairly easy for u/gs. I'd aim for a 777 or 333 out of YYZ, (although check the loads first), and then contemplate doing a same day change if you see space available on a different flight.
And welcome to FT.