Finally, the Southern QRA jets were scrambled just as the last Green Flag sorties were landing. I've seen this practised many times before and also monitored the exercise on the radios. These departed quickly (usually a practise QRA does involve some waiting prior to departure), with other traffic being held off, and only just after the emergency Saudi aircraft above had cleared the runway. With all that in mind, when the QRA came up on London mil attempting to make contact with what seemed like a commercial airliner I was pretty much convinced it was a live shout rather than a practise. Hearing the QRA jets issue the standard warning several times without the target aircraft obeying, and a warning that they would be brought down if they did not comply got my full attention. Eventually, they got the aircraft established on an approach into Stansted (the usual highjack/suspect aircraft diversion for the UK) and followed him down and overshooting, before confirming it was actually an exercise and the target was a Cobham Falcon simulating an aircraft requesting political asylum. A second run through of the exercise actually had the QRA simulating shooting down the target and crashing into London. To hear them run through all that was actually quite sobering.