Bagso, numbers aren't your strength..... Air Canada mainline have only four daily flights on LHR-YYZ as well as Rouge on LGW, EDI and GLA-YYZ but a quick glance at the date suggests peak summer so not enough empty seats to reschedule an existing flight I suspect. Rouge wouldn't have managed a recovery any better than mainline, also their aircraft are maintained by the same people.
Had they scattered the passengers via Lufthansa, BA and Aer Lingus asap to connect on next days varied AC flights with spare seats across europe it might have been better but they didn't think that was the way to go. Hindsight sucks.