> anti air missiles being installed in random flats
The reference to a "High Velocity Missile system" implies that the weapon in question is the Starstreak HVM system (http://en.wikipedia.org/wiki/Starstreak_%28missile%29). Starstreak is a man-portable weapon, so nothing would need to be "installed" anywhere; it'd be a couple of guys standing on the roof with a Starstreak tube.
> triggering those missiles
As far as I can tell, the only way to "trigger" a Starstreak launch is by physically pressing a button on the launcher, so the risk of remotely triggered launch is pretty low. And even if a terrorist or group of terrorists managed to somehow neutralize the Starstreak operators, seize control of the weapon and launch it, the missile itself is so small it wouldn't really matter; the backblast from the launch is negligible (it has to be, for the missile to be safe to launch from the operator's shoulder), and the warhead is just three darts with around 16 ounces of explosive in each. The terrorists would do more damage if they stole bowling balls and dropped them on people from the roof.
> uselessly expensive
Presumably the Starstreak missiles in question have already been bought by the British army, so the only expense would be the cost of having the guys stand around on the roof all day. And since those guys are soldiers the British government is going to be paying them that day regardless of where they happen to be standing.
Re: triggering the missiles, did you read the article? It talks about a scenario where the soldiers are tricked into firing the missiles.
Re: doing more damage if they dropped bowling balls from the roof, that's just flat out wrong. For a start, the missiles travel at 2800 miles per hour which is an order of magnitude higher than the terminal velocity of a bowling ball, so the impact alone would be much more damaging. Secondly, the warhead is housed in a tungsten shell which is designed to fragment on impact to maximise damage inside the target. And then there are the explosives as well. The source for all this, by the way, is the wikipedia link you posted.
I am not a terrorist, but if I was and I would get hold of an anti-air missile I probably wouldn't use to shoot people on the ground. There's plenty of airliners over London at any time. The missile seems to have a service ceiling only 5km but it's still fairly big risk. Couple of guys with guns can't shoot down an airliner unless you give them a Starstreak
(1) presumably the airspace over London is going to be pretty thoroughly controlled during the Olympics, reducing the risk; and
(2) if terrorists want to shoot down an airliner with a man-portable air defense system (MANPADS) like Starstreak, there's easier ways for them to get their hands on one than by infiltrating one of the most heavily secured areas in the world and wresting one out of the hands of professional soldiers -- MANPADS have proliferated widely, and can be acquired on the black arms market for a few thousand bucks (see http://www.fas.org/programs/ssp/asmp/MANPADS.html).
The reference to a "High Velocity Missile system" implies that the weapon in question is the Starstreak HVM system (http://en.wikipedia.org/wiki/Starstreak_%28missile%29). Starstreak is a man-portable weapon, so nothing would need to be "installed" anywhere; it'd be a couple of guys standing on the roof with a Starstreak tube.
> triggering those missiles
As far as I can tell, the only way to "trigger" a Starstreak launch is by physically pressing a button on the launcher, so the risk of remotely triggered launch is pretty low. And even if a terrorist or group of terrorists managed to somehow neutralize the Starstreak operators, seize control of the weapon and launch it, the missile itself is so small it wouldn't really matter; the backblast from the launch is negligible (it has to be, for the missile to be safe to launch from the operator's shoulder), and the warhead is just three darts with around 16 ounces of explosive in each. The terrorists would do more damage if they stole bowling balls and dropped them on people from the roof.
> uselessly expensive
Presumably the Starstreak missiles in question have already been bought by the British army, so the only expense would be the cost of having the guys stand around on the roof all day. And since those guys are soldiers the British government is going to be paying them that day regardless of where they happen to be standing.