'Mulling over to shoot down a probably innocent airliner over London' -
This must be the toughest one ever, but to be brutally frank if it looks like being steered at a target - and one has the means to shoot it down - a lot of fluttering pieces will of course be horrendous, but not as much of an impact, physical or tactical, as a high velocity airliner ?
Shooting one down over the sea or open countryside is an option if you are pretty certain the aircraft is about to be used for a kamikaze style attack, but not over a major city - the damage and loss of life would be greater than allowing the aircraft to hit its target.
However, as said, if the aircraft only became a suspect while already over London, there is no way it could be intercepted in time. Blair always was a compulsive bull****ter and egomaniac, and this is just another example of it.