Simplify the following text for a Grade 11 native English-speaking person.
Original: Since 1399 , the Kings and Lords of Mann existed in a position of feudality as vassals of the reigning King of England , and subsequently of Great Britain and the United Kingdom , who was the ultimate sovereign of the island .
Simplified: Since 1399 , the Kings and Lords of Mann were feudal subjects of the reigning King of England , and then of Great Britain and the United Kingdom , who was the ultimate sovereign of the island .