Big Ben

There is a lantern above the bell tower - Aryton Light.

An excerpt from the article 22 facts about Big Ben

This feature was added in 1873 by Acton Smee Aryton, then First Commissioner of Public Works and Buildings. This lantern is lit when the House of Commons is sitting after dark. Its light is visible from all over London. The Aryton Light was originally shined towards Buckingham Palace so that Queen Victoria, looking out her window, could see when the House of Commons was working.