The Union Flag was the flag of the United Kingdom.
Rose Tyler wore a shirt with the flag on while visiting wartime England. (DW: The Empty Child/The Doctor Dances) She later corrected Eddie Connolly's mistake of calling it the Union Jack; a title only applied while present at sea. Rose also corrected his positioning of the flag. (DW: The Idiot's Lantern)
The Union Flag was often flown atop of government buildings, or from a flagpole. (DW: The Christmas Invasion, Human Nature)
During World War II, Ironside Daleks had an image of the Union Flag in place of their recognition code. (DW: Victory of the Daleks) People often decorated streets bomb shelters with the flag (DW: The Empty Child)
When addressing the nation in broadcasts, the Prime Minister of the United Kingdom would sit at a desk between two Union Flags. (DW: The Christmas Invasion)