Academic, I'm confused, why did you get a domestic flight if you wanted YYZ-LHR?

Anyway, I do think it's worth trying to find out how oversold AC856 really is, if you still want it. If it's only a few seats, it's worth calling back regularly in case there are some changeable reservations that get changed (if it's only oversold by 4, say, and you're only looking for 2 seats, they might open up).

andrew