The rear entrance/exit to PINDAR (The UK equivalent to the US PEOC) is allegedly based on one the platforms at Charing Cross Station. It is assumed that it's there to necessitate access to a railway or the Underground if the government needs to escape the capital. It is classed as a "Military Citadel." Others include the conspicuous one at the back of Horse Guards/The Mall, the abandoned one at Dollis Hill, the old Cabinet War Rooms under the Treasury, and what used to be called Q-Whitehall.
The Boeing 747-200 (G-BDXJ) used as Air Force One at Stansted Airport was used in Casino Royale (2006) as the SkyFleet S570. The aircraft was not modified back to standard configuration after Casino Royale. In several scenes, B-52 style engine pods and fuel pods on pylons are visible. The aircraft is actually located at Dunsfold Aerodrome in Surrey, England, and is not airworthy. It can also be seen in many episodes of Top Gear (2002).
"Stansted Airport" is portrayed by Dunsfold Aerodrome in Surrey, England, which is also home to the Top Gear studio and test track. Air Force One is parked on the section of runway between the Bentley and Chicago corners of the Top Gear test track. Multiple tyre tracks, produced by Power Laps conducted by Top Gear's "The Stig," can clearly be seen on Bentley Corner.
In this movie, Aaron Eckhart plays President Asher whom Gerard Butler (who plays Mike Banning) must protect. Banning is considering resigning before a catastrophe compels him to stay at the end. In Battle Los Angeles (2011), Aaron Eckhart plays the main character who was considering retirement until a catastrophe compels him to stay at the end as well.