Great advice from KLondon...one other caution to be aware of is that upon landing in Toronto, you may have to re-claim checked baggage and then carry it thru US Customs (and then upon clearance with customs, effectively re-check it). I needed to this a few years ago and it was horribly confusing/time-consuming.
I would double-check with the Air Can agents in London when you leave exactly how to choreograph your moves at Toronto Airport with checked bags, shuttles, customs etc...
I would add that the chaos in Toronto Airport is terrible....hopefully all the reconstruction they're doing will help, but my gut feeling was "Maybe they should have started from scratch and built a whole new airport at a different site...Pittsburgh did this and it's such a pleasure to experience connecting thru Pittsburgh Airport! Back to Toronto, I landed at Terminal 3 last month and forced hordes of crowds in the terminal and right outside the terminal...traffic of shuttle buses/airport drop-offs was horribly choked....there literally wasn't enough room to safely do your thing to get away from the airport....I was told later that this was the "new" terminal!!! Yikes!
Suggestion: In the future, try Buffalo Airport or Detroit Metro Airport....they're both new (Detroit's brand-spanking new!) and they're very user-friendly....plus you avoid the airport customs crunch this way!