Trim Stab, you seem to be reducing this event and your speculation to the lowest possible level. How could your imagined response to an order to comply affect a supposed engagement order?
Sorry - I don't understand your point, but if I am interpreting your gobbledygook correctly, I think you may be assuming that the next aerial terrorist event will be an exact copy of the last.
I am just considering different possible scenarios that could unfold. For example, the threat aircraft could announce that they intend to fly into the Palace of Westminster, but that they don't want to cause any casualties and will give the occupants time to evacuate. So then what happens? Do you shoot down the aircraft and risk ground casualties? What do you do if an aircraft is in IMC, squawking 7600, and correctly following its planned route to destination, which happens to go over central London? Or an aircraft on a route over London squawks 7700 and makes an emergency descent? Or a modern plastic aircraft with a very low radar signature just turns the transponder off completely?
I have flown one such aircraft as a UAV simulator on a military exercise and I know that they are not easy to see on the radars of some very modern air defence aircraft.