If you arrive in T3 and need to connect to a Canadian destination (in T3), you will have to go through a security check, after clearing customs, to do this i.e LGW-YYZ-YYC (Westjet). On the other hand, if you arrive in T1 with a connection to a Canadian city, there is no security check after clearing customs i.e. LHR-YYZ-YYC.
The discussion seems to have devolved into mixing YVR (original entry point) arrivals and YYZ arrivals.