LHR has this "conformance" system where they won't even allow you to go through security if you present yourself less than x minutes before departure, no matter if you could still make it. Last year, at the entrance of the fast lane security, I overheard an UA agent pleading the case of an elderly couple that was past conformance time. The LHR security officer gave the UA agent a stern "no" and she sighed and took them back to the check-in desk (presumably) to rebook them, suggesting even the airlines have few if any control over this system.
I'm not a fan of this system and definitely hope YYZ isn't making this a commonwealth thing.