. An operating air carrier shall not be obliged to pay compensation in accordance with Article 7, if it can prove that the cancellation is caused by extraordinary circumstances which could not have been avoided even if all reasonable measures had been taken.
The critical wording is "all reasonable measures" and how that is interpreted.
Also the contract is for a specific flight so the previous flights should have no bearing on that contract. For example, if your 747 has an engine failure going in to SYD it would be reasonable to expect BA to have another aircraft available at LHR to take over the flight the SYD aircraft would have rotated in to.
It would be reasonable to assume that an airline as big as bmi baby, given its relation to bmi and LH, would have a spare aircraft to cover for reactionary cancellations. bmi always had a service backup available; Ryanair have spare aircraft at selected bases to provide backup, why not baby?