A connecting flight improves your chances considerably. Coming in from LHR will move you way up the upgrade list in YYZ. I'd confirm the LHR leg now as you're starting journey there and won't get the same advantage if the R space dries up.

Connecting brings you above at least those on status originating and perhaps even above the originating SEs on day of flight upgrades. Not fair for those of us that live in yyz, but the way it works.