This is the building off Covent Garden which was used as the MI5 building in Spooks. (I speculate that it was originally used as a double for the former M15 building in Curzon Street, the only building in London with a machine gun emplacement built into the design; sadly now replaced with an office block). Apparently the interior was also used for numerous TV things such as Poirot.
It is fully open to the public and I took a free guided tour.
It's an amazing Art Deco building with incredible decorations internally. It survived the Blitz intact and every inch is manicured. There are also some very interesting historical artefacts, letters, paintings, etc.
You don't have to know anything about Freemasonry to appreciate the building!