Buckingham Palace

Buckingham Palace was the London home and primary residence of the British monarchy.

21st century
In 2008, during the reign of Elizabeth II, most residents of London left the city out of fear over an alien invasion. Elizabeth II herself chose to stay behind. A starship fashioned after the Titanic was about to crash into the palace until the Tenth Doctor, at the controls, managed to avert the disaster. (DW: Voyage of the Damned)

The TARDIS was parked in the grounds of Buckingham Palace prior to the Tenth Doctor being tranported to San Helios. According to the Doctor, the Queen "doesn't mind". (DW: Planet of the Dead)

Alternative timelines
In one timeline, the Titanic did crash into the Palace, causing a nuclear explosion. (DW: Turn Left)

In an alternate timeline where all of history occured at once after River Song refused to kill the Eleventh Doctor, Buckingham Palace was the home of Holy Roman Emperor, Winston Churchill. He returned to the palace on his mammoth from a conference in Gaul with Cleopatra. (DW: The Wedding of River Song)

Behind the scenes

 * According to his book Doctor Who: The Writer's Tale, Russell T Davies' the original draft of Voyage of the Damned had the Titanic demolish part of the palace, leading the Queen to curse the Doctor (continuing the recurring recent theme of the Doctor making enemies with British royalty), but this was dropped as too negative for a holiday special. His later script, Turn Left, did allow the ship to destroy the palace, and most of southern England, but in an alternate timeline.