One C seat on LHR-YYZ is not that difficult if done reasonably far ahead in my experience, and if not concerned particularly about which flight. Coming back the day flight tends to be available last.

Getting 2 C seats though is much more difficult - flights via YOW, YUL or YHZ are alternatives worth checking for this though. Booked 2 C seats YYZ-YOW-LHR recently for mid-June.