UK Flag History
The United Kingdom of Great Britain and Northern Ireland flys the "Union Flag" or, as it is more popularly called the "Union Jack." It is easily distinguishable from other flags by ist double cross design and blue background. The flag has been used in a variety of forms since 1606, when the flags of the Kingdom of England and the Kingdom of Scotland were first merged to symbolise the "Union of the Crowns," which occurred in 1603. The current design of the Union Jack dates back to 1801 and the union of Ireland and Great Britain. The flag remains unchanged even after the partition of Ireland in 1921 and the creation of the Irish Free State.
The large red cross, edged in white, centering the flag is the the cross of Saint George, the patron saint of England. It is superimposed on top of the diagonal red cross of Saint Patrick, the patron saint of Ireland. Both are are superimposed on the Saltire of Saint Andrew, the patron saint of Scotland.
Prior to the Union Jack, the flag flown in England was that of St. George's Cross. A relatively simple flag with a white background and the red cross of St. George centering it. The exact origins of the flag are unclear, however, it is known that the flag appeared during the Middle Ages. The first known record of the use of St. George's Cross as an emblem of England is related to an account of the Welsh War of 1275.
One theory suggests that the flag was born from the Crusades. St. George's cross was already associated with England at the start of the Crusades and the Pope made the decision that English crusaders would wear a white cross on red, French crusaders a red cross on white and italian crusaders a yellow cross on white. The English traded with their rivals, the French in January of 1188, so that they could don the red cross with white background.
The Union Jack incorporates St. George's Cross in the design, the flags of Scotland (below / right) also played a large role in the look of the Union Jack today. While both of these flags have been incorporated, recently it has been argued that the flag does not represent all of the members of the United Kingdom as the flag of Wales(left) is not represented on the Union Jack. Other private citizens have argued that the flag is racist as it does not represent all communities of the United Kingdom. The overwhelming response to this sentiment is that the flag is not related to racial tension and should not be changed.
In addition to these flags, the Royal Standard is used by Queen Elizabeth II in her capacity as sovereign of the United Kingdom (left). There are two different standards in use, the one shown to the left is flown throughout the UK with the exception of Scotland. The Royal Standard used in Scotland is shown below. While universally the flag is called a "standard," the flag used in the UK is a "banners of arms," due to the shield of the Royal Arms. Outside of the UK , the flag is comprised of a blue disc containing a wreath of gold roses encircling the crowned letter "E."
The Royal Standard is flown when the queen is residing at one of the royal residences, on the queen's car during official journeys and on aircraft before and after flights. It also may be flown on any building during a visist by the queen. Westminster Abbey, a "royal peculiar," is the only church permitted to fly the Royal Standard without limitation.
When the Union Jack is flying over Buckingham Palace, it means the queen is not in residence. Similarly, when the Royal Standard of the King of Scots is flying above Holyrood Palace or Balmoral Castle, it also means the queen is not in residence.
The Royal standards incorporates the flag of scotland, as seen to the right of the above paragraph, and also the irish harp as seen on the historic flag of ireland(below / right).
What is interesting about the transitions the flags of the United Kingdom have undergone is that they still don't have an official flag. The Union Jack is technically the flag of the Monarchy and has become the flag of the United Kingdom only through popular use. Therefore, we may not have seen the end of the changes to the flags of the United Kingdom as all one would need to do to change the flag is make one that is more popular than the current Union Jack.
1 - Wikipedia Flags of United Kingom
2 - Wikipedia Flags of England
3 Wikipedia Royal Standard (United Kingdom)