Big Ben

Inside the tower there is the so-called Prison Room.

An excerpt from the article 22 facts about Big Ben

It is an oak-paneled prison chamber, accessible only from the House of Commons. The room was last used in 1880, when atheist Charles Bradlaugh, newly elected Member of Parliament for Northampton, was imprisoned by the Sergeant-at-Arms after protesting against taking a religious oath of allegiance to Queen Victoria. Officially, a sergeant-at-arms can still make arrests, as has been the law since 1415. However, the room is currently used by the Committee on Petitions, which oversees petitions submitted to Parliament.