In John Le Carré's espionage novels, the British Secret Intelligence Service is headquartered at Cambridge Circus in London, where Shaftesbury Avenue and Charing Cross Road meet. For this reason, the HQ is known throughout his novels as "the Circus". Whether or not the author intended this nickname to be a wry comment on the smooth and efficient running of British Intelligence is unknown.
In real life, SIS (also known as MI6) was at the time of this novel based in St James's Street and Broadway, moving in 1966 to Century House in Lambeth. It now occupies a rather grander headquarters at Vauxhall Cross. All that you're likely to find at Cambridge Circus is the Palace Theatre and a lot of traffic.