I'm pretty sure something will happen.

The reason is all the security and military.

One of all the missiles around town or something will be triggered. A plane losing radiocontact at the exact wrong moment will get shot down hitting downtown London. Or some military nuts will go overboard with visitors tired of the checkpoints.

Not only will this olympic turn into a huge disappointment when it comes to the number of spectators, it will also be a huge PR disaster.

Anyway, whatever happens, I'm quite sure that they will do whatever they can to pin it on Iran or some other nation that they want to get rid of. Partly because they will never be able to admit that their over-the-board security paranoia, caused whatever happens.