Surface to where?
Presuming then that the debris of a shot down passenger plane doesn't hit the Olympic Stadium itself, is it any more acceptable that the damage is spread over a wider area rather than one specific place? People on the ground will die, and buildings will be destroyed. If it is a passenger airliner that's hijacked, then the bodies of those on board will be spread over a large area, requiring first a massive investigation and then clear-up effort, making it highly unlikely that the games could continue until the work was completed. This of course is if the missiles are any use at all, and successfully intercept a hijacked aircraft. When deployed in the Falklands, the Rapier missiles scored only one confirmed "kill", while the Starstreak HVM's due to be sighted in Leytonstone have never been used in battle.
The reality is that since 9/11 al-Qaida has shown very little to no interest whatsoever in reprising that attack, reasoning that it takes a significant amount of time to train those involved in the plot to fly, with no guarantee that their mission will be successful due to increased security. Hence why there have been multiple attempts instead to bring down aircraft, either with bombs concealed in shoes, underpants, liquids or printer cartridges, all of which have nonetheless also failed or been disrupted. There is no reason whatsoever to believe that al-Qaida or any other group has done a volte face and decided hijacking is the way to go again, nor is there any intelligence to suggest that they're thinking on a smaller scale, either hijacking a helicopter or small plane and filling that with explosives, or using some sort of airborne transport to launch a Mumbai-style attack.
Why then is the government so determined to put anti-air batteries up when their only use would be to divert the death and destruction from one area to another? One suspects it's the same reasoning that lies behind there being more military personnel around Stratford than there are in Afghanistan: firstly that there's not much point paying soldiers to sit around doing nothing, or buying weapons only to leave them unwrapped at bases, and secondly that they genuinely seem to believe that people are "reassured" by their presence. While some might be content with squaddies conducting pat-downs, the idea that putting surface to air missiles on various tower blocks does anything other than scare people in general and particularly worry those living in the surrounding area is laughable. It's the action of a security state that believes more in the illusion of safety than in say, protecting or warning about the more viable threat of a small group of men armed with automatic rifles. Or indeed, with trying to ensure that there isn't another bout of rioting, which is about a million times more likely than any terrorist attack. That though would require a complete change in government policy, rather than just a scaling down in the level of security lunacy.