(covers information from several alternate timelines)
Old Britain was a historical region of Earth also known as Great Britain (abbreviated GB) and the United Kingdom (abbreviated UK), which contained England and Scotland.  The country played a significant role in Earth history. Residents of Britain were known as British, Britons, or Brits.
Its flag, known as the "Union Flag,"  was two red crosses superimposed on a white cross on a blue background, and which was also carried on the fighting vessels of its Royal Navy, as was evidenced on the prow of Geordi La Forge's model of HMS Victory. (ENT: "First Flight", "Storm Front"; TOS: "The Trouble with Tribbles"; TNG: "Elementary, Dear Data"; DS9: "Our Man Bashir")
Great Britain was an instrumental participant of World War II against Nazi Germany, particularly in the Battle of Britain. (DS9: "Homefront") In 1941, President Franklin D. Roosevelt of the United States of America pressured the United States Congress to send aid to the British. (TNG: "The Big Goodbye")
Alternate, parallel, and imaginary Britains
In an alternate version of 1944, caused by activities within the Temporal Cold War, after England was taken over by Nazi Germany, Prime Minister Winston Churchill urged all Britons to continue to fight for either anarchy or order in World War II. These events were witnessed by Enterprise NX-01. (ENT: "Storm Front")
In the holoprogram Julian Bashir, Secret Agent, set during the Cold War, the government of Great Britain paid for every item and luxury that secret agent Julian Bashir would need for his espionage and intelligence gathering duties. Garak was incredulous that a government would even pay a spy for trivial items such as an apartment and weapons. (DS9: "Our Man Bashir")
- Cambridge University
- English Channel
- Glamis Castle
- Loch Ness
- Sherwood Forest
In the real world, the "Kingdom of Great Britain" merged with Ireland in 1801 to become the "United Kingdom Of Great Britain And Ireland" (or UK), later called the "United Kingdom Of Great Britain And Northern Ireland". The term "Great Britain" was used once, in "Our Man Bashir". It was referred to as "Old Britain" by Spock in "The Trouble with Tribbles".
Dialogue from "Our Man Bashir" and "The Trouble with Tribbles" implies that Britain no longer exists as a sovereign country in the 23rd and 24th century. It may have lost its sovereignty by joining the European Alliance or United Earth. However, this may depend upon the internal political functioning of both the United Earth and the European Alliance.
The script of the Star Trek: Deep Space Nine episode "To the Death" described the position of the USS Defiant's security officers facing the Jem'Hadar on the transporter platform as being "like British infantry". Later in the script, the rogue Jem'Hadar's weapons were described as being "like a cross between the halberds used by British beefeaters and Japanese naginatas."
- "Our Man Bashir" called this nation "Great Britain". Great Britain was actually the name of the island that consisted of England, Scotland, and Wales. The latter has never been mentioned in a Star Trek production, with the only reference being the proper noun Welsh rarebit, although the Welsh language has been featured. Lieutenant Rhys' last name also suggests Wales ancestry. The full and correct name of the country, the "United Kingdom of Great Britain and Northern Ireland", was never mentioned, although the abbreviation "UK", for "United Kingdom", was seen on an okudagram. The United Kingdom in fact extends beyond Great Britain, also containing Northern Ireland.
- The Union Flag is better known under its popular name, the "Union Jack", and is actually an amalgam of the flags of three of the Union's constituent nations: the Scottish "Cross of Saint Andrew" flag (white X-shaped cross on blue field), the Irish "Cross of Saint Patrick" flag (red X-shaped cross on white field) and the English "Saint George's Cross" flag (red cross on white field). The flag traces it origins back to 1606, when Scotland was united with England, but only became final and officially adopted in 1801, pursuant the formation of the United Kingdom of Great Britain and Ireland.