The SIS Building at Vauxhall Cross has been the headquarters of the Secret Intelligence Service, popularly known as 'MI6', since 1994 - and quickly became one of London's most recognised buildings, featuring in many movies. Obviously, it is not open to the public and unexpected visitors are discouraged.
From the website of the SIS: "At the Secret Intelligence Service (SIS) – otherwise known as MI6 – our mission is clear. We work secretly overseas, developing foreign contacts and gathering intelligence that helps to make the UK safer and more prosperous. We help the UK identify and exploit opportunities as well as navigate risks to our national security, military effectiveness and economy. We work across the globe to counter terrorism, resolve international conflict and prevent the spread of nuclear and other non-conventional weapons. We are here to help protect the UK’s people, economy and interests."