The British Flag is called the Union Flag. Many call it the Union Jack although some people think this is only when it is flown from the Jackstaff of a ship and are often quick to point this out to those who use it in everyday life.
However, this too has been disputed and the Admiralty in 1902 stated that the term Union Jack should not be solely reserved for when it is flown from the Jackstaff.
Also, the original Union flag was for when James I and VI came to the throne in 1603. The current version with the Cross of St Patrick is from 1801 when Ireland were incorporated into the Union, something Disney didn't realise when they made Pocahontas.