The Security Service MI5 headquarters is called Thames House, and is at 11 Millbank, by Lambeth Bridge. This intelligence agency deals with Counter-Intelligence operations against foreign spies and also Counter-Terrorism against foreign and domestic terrorist suspects.
The southern part of the complex, 12 Millbank nominally contains the Northern Ireland Office, but it is inconceivable that much of this southern building, is not also used by MI5.
Thames House has been recently Designated as a Protected Site under the Serious Organised Crime and Police Act 2005 section 128, which makes it a criminal offence to cross the outer boundary without permission, Strangely this boundary specifically exempts the main steps in front of the main entrance which is set back from the road. The pavement outside is part of the SOCPA section 132 Designated Area around Parliament Square, where you can be arrested for, in the opinion of a Police constable, holding a demonstration without having obtained prior, written permission a week before. The word "demonstration" is not defined so it could mean just wearing a tshirt with a slogan which the Policeman does not understand.
These restrictions on demonstrations have nothing to do with the "security" of Parliament, and everything to do with suppressing sometimes noisy political demonstrations, which remind the Labour Government of their failures over the Iraq war and other controversial policies which they have inflicted on us.
Perhaps the steps of the MI5 Thames House building are a legal Temporary Autonomous Zone.
Click on the image above to go to Google Street View, which passes along the eastern side facing Millbank and the northern side facing Horseferry Road, giving only longer range views of the Thames House building along its Thorney Street western and southern sides.
Ground level and aerial shots of Thames House have featured heavily in the BBC Torchwood sci fi drama Children Of Earth, screened on 5 consecutive nights in July 2009. The "13th floor" is where a evil extra-terrestrial monster known as the "356" installs itself as an "Ambassdor" via a "pillar of flames" descending from the sky.
See:the Spook Country blog article: Torchwood Children Of Earth - MI5 Thames House and the Government Car and Despatch Agency