We (two people) did LHR-YYZ earlier this month. We booked it some months in advance and at that time there were two C seats available on most flights. Availability on onward flights within N America was more limited and we had to accept one seat in Y (which was upgraded on the day of travel in the YYZ lounge).

Obviously, though, what was true for a weekday in mid-January cannot be assumed to apply to peak travel periods.