LONDON — More often than not, the Olympic host city doesn’t really host the Olympics. It merely hosts a couple of IOC caviar buffets, while the real event tends to take place in a remote pasture or distant slum. That’s not so of the London Games, an exercise in bravado that is really here, in the heart of this great, ageless swirl of a city, with all the potential chaos, headache and magnificence that might entail.