I’d be surprised if QF have any spare aircraft ready to be used incase of irrops. They’ll happily put pax onto other scheduled flights and perhaps shuffle flights if a problem so they can “create” a plane, but having one sitting round just waiting to be used - I doubt it.

Wasn’t there a rumour that QF positioned a second 789 to PER so it could substitute on the inaugural PER-LHR flight if the chosen frame failed to proceed? Maybe the only case in a long time of ensuring a flight left as planned.