King Arthur's Britain
When Constantine III revolted in England in 406 AD and took all its Roman troops with him to France, that was really the end of Roman control in Britain. The British wrote to the Romans asking them to come help out in the 420s, and got back only a letter advising them to stop paying taxes and use the money to organize some soldiers, because now they were on their own.
National Geographic about Arthur
There is a story, which you have probably heard, that at this time there arose in England a famous king, King Arthur, who had a famous magician helper named Merlin, and whose Knights of the Round Table fought off the Anglo-Saxons and the Danes, and kept England civilized and unified. It is hard for historians to say definitely whether Arthur really existed. But it does seem likely that the English organized an army to keep the Anglo-Saxons and the Danes out.
However, as the Arthur stories say, the effort failed. By the 600s, England had been taken over by the Angles and the Saxons. The name England means the land of the Angles, and English is the language that the Angles spoke. The Angles and Saxons did at least manage to fight off the Danes for the most part.