This is the Dismounting Ceremony at Horse Guards (or 4 O’Clock Parade), which takes place everyday in that little courtyard behind the horse boxes. A smart officer comes storming out of the gate with his big boots making a metallic slapping sound on the concrete and goes down the line of Foot Guards inspecting their hats and haircuts and the shine of their shoes, etc., making sure that they all look respectable. Once he’s happy he orders them to sheath their swords and head inside.