 By the year 600 , most of what is now England had been conquered by invaders from the continent , including Angles , Saxons , and Jutes . Bernicia and Deira , the two Anglo @-@ Saxon kingdoms in the north of England , were first united under a single ruler in about 605 when Æthelfrith , king of Bernicia , extended his rule over Deira . Over the course of the 7th century , the two kingdoms were sometimes ruled by a single king , and sometimes separately . The combined kingdom became known as the kingdom of Northumbria : it stretched from the River Humber in the south to the River Forth in the north . 
