Question: how was the first king of england decided

title: Kingdom of England
text: The standard title for monarchs from Æthelstan until John was Rex Anglorum ( `` King of the English '' ) . Canute the Great , a Dane , was the first to call himself `` King of England '' . In the Norman period Rex Anglorum remained standard , with occasional use of Rex Anglie ( `` King of England '' ) . From John 's reign onwards all other titles were eschewed in favour of Rex or Regina Anglie . In 1604 James I , who had inherited the English throne the previous year , adopted the title ( now usually rendered in English rather than Latin ) King of Great Britain . The English and Scottish parliaments , however , did not recognise this title until the Acts of Union of 1707 . 

Answer: answer: The standard title for monarchs from Æthelstan until John was "King of the English" until Canute the Great called himself "King of England ''.  The term eschewed all other titles John's reign onwards.
answer: The standard title for monarchs from Æthelstan until John was "King of the English" until Canute the Great called himself "King of England '' for the first time.