There is quite a large police presence inside the Palace of Westminster which is under the direction of the Serjeant at Arms.
Their duties include ensuring free access to the House for MPs and peers, if necessary holding up the traffic for this purpose.
During the Speaker's procession, the head policeman in central lobby shouts "Hats Off, Strangers" as the procession approaches, at which fellow policemen and members of the public are expected to remove their helmets or hats.
At the rising of the House, the police shout "Who Goes Home?"
This is a relic from the days when the streets near Westminster were dangerously unlit and MPs were invited to walk together or to hire boats home on the Thames.
The police do not enter the chamber when the House is in session and if it sits beyond midnight, they remove their helmets.