Big Tudor, you are quite correct that the decision making process is more complex than I described, I was just trying to condense the complete thing into two paragraphs
Notably I was writing this having just seen the previous evening exactly what I described, FlyBe at Belfast City cancelling an Edinburgh and transferring pax, who were already in the departure lounge, onto a later flight. Colleague with me was travelling with them to Southampton on the last flight of the day and said it was fortunate it was not his plane which had gone tech, and I had explained to him in the terms I wrote here how you decide what to cancel if the situation arises.
Note too how many of us on this thread are referring to FlyBe, they must surely be coming to the attention of the CAA as being over-extended somewhere along the line and getting AOG rather too often. I hope they get through into a better time.