Of course the crew (and BA ops) know exactly when they time out. But, that is of no moment.

The aircraft is headed to LHR. Thus, it makes sense to get the aircraft (and its pax) as close to LHR as possible. Makes zero sense to cancel at YYZ whent the crew is good for SNN. While the aircraft is enroute SNN, BA can position a replacement crew at SNN and thus continue on to LHR, presumably arriving a great deal earlier -- the goal.

Sounds like a situation well-handled.