It is interesting that everyone seems to be treating this lightly. Given a military system that is being continually challenged and the current terrorist alert state this intercept could have ended very badly. This is a defense/defence of countries from potential attack. For all anyone in the defense/defence agencies knew this was another United 93 over an hour transit into another sovereign country without calling them is not at all clever not even dilatory, this is outright hazarding your aircraft and extremely poor airmanship. I have been on the other side of the situation a few times luckily before 9/11 so the pressure was less but we still had extremely close incidents with fighters having to do visual idents on aircraft that had failed to clear their entrance into the FIR/UIR.
Imagine night poor VMC or even IMC the fighter pilots both flying armed aircraft and both on edge as this is definitely not an exercise, missiles perhaps already locked on to your aircraft. Extremely twitchy as they know this is possibly an airliner full of pax but being told maximum caution it may have been hijacked. How close are you to being headlines for several weeks? If that doesn't get you thinking then it should.
The best option in these cases would be to add the cost of the alert and fighter scramble to the route charges for that airline. I can assure you that one case of that eyewatering cost and none of you would forget to monitor FIR/UIR boundary crossings.
I understand the stupidity of pilots using guard as a chat frequency and the UK practice pan system ought, like the UHF guard, to have a parallel practice frequency rather than clutter 121.5. If you find non-emergency chatter on guard is interfering with monitoring it - do not just switch it off raise an MOR or make an ASRS report.
This could easily have become a case of the crew and pax of the aircraft in question not arriving home. But the fighter pilot and his entire chain of command would have had to live with the actions - think about it.