You can't expect anything better when you operate an aging (to use a polite word for clapped-out) fleet. The aircraft involved was C-FCAB, a 28 year-old 767-375ER. The rest of the AC 767 fleet comprises 12 aircraft 24 to 28 (average 26) years old and two that are a sprightly 16 years old. AC thinks it's probably risky to send out one of these on a rescue mission, even if they could spare one.
The daily Ottawa-LHR flight (AC888) is operated by a 767 and is scheduled to depart at 2240. If that flight is cancelled, the poor sods booked to return to Canada from LHR next day are SOL -- but at least they get plenty of notice.
It's third-world service from a supposedly first-world airline.
Rockhound