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)
In 1941, The Ritz was decorated with numerous Union Flags (TW: Captain Jack Harkness) and during World War II, people often decorated streets and bomb shelters with it. (DW: The Empty Child) Ironside Daleks had an image of the Union Flag in place of their recognition code. (DW: Victory of the Daleks)
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)