"His gun is jammed," the observer states, suggesting that the misunderstanding of the situation here was even greater than it appears.
One wonders how long this beloved British tradition of having the guards ignore tourist antics (to the point of being laid hands on) will continue. The guard is a symbol of the Queen's authority, and a symbol of the state. In the age of terrorism, that makes them a target for those wanting to send a symbolic message of their own. The guard here was righteous and correct in his response.