It later departed back to LHR, albeit quite late. Must have been a fixable problem then, no engine change required:

BRITISH AIRWAYS - BA 92

[Planned Flight Info]
YYZ-LHR 18:55/Sun - 06:55/Mon 07:00

[Flight Notes]
YYZ-LHR MEMBER OF ONEWORLD
YYZ-LHR DEPARTS TERMINAL 3
YYZ-LHR ARRIVES TERMINAL 5
YYZ-LHR 9/ NON-SMOKING
YYZ-LHR ET/ ELECTRONIC TKT CANDIDATE

[Operational Info]
YYZ 20:30 NEXT INFO WILL BE AT
YYZ 22:45 ESTIMATED TIME OF DEPARTURE
YYZ 23:31 LEFT THE GATE
YYZ 23:46 TOOK OFF
YYZ - DELAY OPERATIONAL REASON
LHR 11:11 ESTIMATED TIME OF ARRIVAL