( What has Gone Before. )
368 CE. The Saxons start to be a nuisance in Britannia.
407 CE. The Romans pick up their toys and go home.
410 CE. The Romans tell Britannia to stop pestering them and look after itself.
410 CE. Rome is sacked.
449 CE. The Saxons, Angles and Jutes stop messing about and invade Britain properly.
500 CE. The Roman Empire is history.
( For more details and less flippancy see the Chronology. )