I've flown all four YYZ-LHR routes a few times each and my impressions are as follows:

- I think the easiest upgrade is on the day flight which departs YYZ at 9:00am. Last time I flew the day flight on a weekday the J and Y cabins were both around half full. FA mentioned that loads have been low on this flight and AC was considering cutting it - not sure if this was an idle comment or some serious insight.

- When comparing the three overnight flights, I've never had a problem upgrading on the 11:15pm flight. I've been waitlisted on the 6:20pm and 8:40pm but I've normally been successful.

So, based on my sample size of about three for each flight (i.e. my stats are worth next to nothing) the day flight or latest flight may be best, but it's almost impossible to say. Maybe someone with more stats can comment? In any case, just make sure you call in or do the online upgrade at exactly midnight four days before the flight!