Westminster Palace, home to the British Houses of Parliament, is right on the river Thames. A magnificent Neo-Gothic building dating from 1840, it's most recognizable from the clock tower at one end known as Big Ben. (In fact, Big Ben is actually the bell inside the tower.)
Parliament is made up of two houses: the House of Commons and the House of Lords and both have their meeting chambers inside here. It is possible to sit and watch from the Visitors' Gallery if you like seeing grown men taunting each other with bad jokes. Once a year, the Queen puts on her crown, sits on her Throne in the House of Lords and officially opens Parliament.