The national flag of the United Kingdom is the Union Jack, also known as the Union Flag. The current design of the Union Jack dates from the Act of Union 1801 which united the Kingdom of Great Britain and the Kingdom of Ireland (previously in personal union) to create the United Kingdom of Great Britain and Ireland.
The flag consists of the red cross of Saint George (patron saint of England), edged in white, superimposed on the Cross of St Patrick (patron saint of Ireland), which are superimposed on the Saltire of Saint Andrew (patron saint of Scotland). Wales is not represented in the Union Flag by Wales's patron saint, Saint David, as at the time the flag was designed Wales was part of the Kingdom of England. Source: Wikipedia
The earliest form of the flag of Great Britain, developed in 1606 and used during the reigns of James I (1603–25) and Charles I (1625–49), displayed the red cross of England superimposed on the white cross of Scotland, with the blue field of the latter. Because in heraldry a red on blue is not considered permissible, the red cross had to be bordered with white, its own correct field.
During the Commonwealth and Protectorate period (1649–60), the Irish harp was incorporated in the flag, but it resumed its original form on the Restoration of Charles II in 1660. It continued in use until January 1, 1801, the effective date of the legislative union of Great Britain and Ireland. In order to incorporate the Cross of St. Patrick (a red diagonal cross on white) while preserving the individual entities of the three crosses, the heraldic advisers to the sovereign found an elegant solution.
The existing white Cross of St. Andrew was divided diagonally, with the red appearing below the white on the hoist half of the flag and above it on the fly half. To avoid having the red cross touch the blue background, which would be contrary to heraldic law, a fimbriation (narrow border) of white was added to the red cross. In the centre, a white fimbriation also separated the Cross of St. Patrick from the red Cross of St. George. Source: Britannica