When Did Saint George Become Patron Saint Of England

In 1415 after the English victory of Agincourt , during the reign of Henry V.
Before this date - in -1348 King Edward Ill had established the Knights of the Garter, the oldest order of Chivalry in Europe.
The Order of the Garter was dedicated to the Blessed Virgin Mary, Edward the Confessor and St George.
The Insignia of the order  of the Garter consists of a collar and badge appendant which is known as the George.The Star, the Garter and the Sash with the Investment Badge  is called the lesser George. The latter is a gold and richly enamelled representation of St George on horseback in the act of slaying the dragon.

John Cabot  flew the pennant of the Cross of St. george when he sailed to Newfoundland in 1497.It t was also flown by Sir Francis Drake and Sir Walter Raleigh.
The Pilgrim Fathers flew the flag of St. George when they arrived in Plymouth Massachusetts on the Mayflower n 1620.
It is  the flag of the Church of England.

How did saint george become a saint?
  
I would like to know because i have a project to do on him for college