"God forbid we have to use these missiles - it's going to be a split-second decision between killing everyone on the plane, and letting the pilot/hijacker kill everyone on the plane and lots of people on the ground. This is an absolute last resort, after jet fighter intercepts and helicopter-mounted snipers have failed."
Nah, the choice isn't that. The hypothetical plane would fall on people in either scenario, the question is WHO would it fall on?
The Rapier missile system is somewhat old (1970's tech). The missile itself, whilst reliable, has a flight time of ~13s at approx 650m/sec.
Now, I think London is a bit bigger than that, so let's go with the scenario of a plane load of terrorism being aimed at the Dome during the opening ceremony.
Plane detected, missile fired, where do you think the plane will come down? Yeah, somewhere in central London. In bits. All over the place.
So, who are those missiles protecting exactly? Well, at least they protect the attendees, you may say, and that's a good thing. So, let's take a look at who those attendees are shall we, as we know its not us mere citizens who have to fight in an auction over the remaining <~30% of the tickets that were not pre-reserved (yeah, we only get to pay for the Olympics with our taxes, we don't get a right to attend it or buy a ticket. Never mind, we can use it to make us feel better about Austerity, eh? Like we did for the Royal Wedding that we also paid for...)
So, who's going to be the target for the plane? Well, let's see here....oh! It would appear to be a collection of rich people, government cronies, heads of corporations, representatives of foreign powers (including Iran and Syria, no less!) and various other pals of our government.
Seems we're quite happy to use weapons to protect these people at the expense, both in terms of finance and, potentially the lives, of the citizens of London and, indeed, Britain. (And I've not even *started* on the vile "social policies" being used to "clean up" the image of the area around the games before our foreign "dignitaries" arrive!).
Boycott the Olympics, I say.
Rapier Missile Stats: http://en.wikipedia.org/wiki/Rapier_(missile)