The Balliwick of Jersey is a semi-independent country European country. Jersey, Guernsey, Alderney, Sark, Herm are former feudal fiefdoms in the English Channel. They were ruled by William the Conqueror, the Norman Duke of Normandy, prior to his conquest of England in 1066. As such William, and subsequent monarchs, remained sovereign over these fiefdoms in his or her capacity as Duke or Duchess of Normandy -- not as sovereign of England, or later Great Britain or the United Kingdom.
Jersey had different laws than the United Kingdom, particularly different tax laws, making it a popular place for wealthy UK citizens to retire.