The Romans, in a word. And before them, the independent tribes. When the Romans left there was something of a free-for-all until a few powerful warlords started to cement their power.

If you want to know about the era and you like old stuff, I recommend you pick up The Venerable Bede’s famous book The Ecclesiastical History of the English People. Despite the stultifyingly-dull-sounding title, his book is great. It’s the stories of the end of Roman occupation in Britain, of king Arthur, and the coming of Hengest and Horsa and the Saxons. They’re stories, of course (there’s no evidence for a King Arthur of the sort we think of when we think Sword in the Stone) but Bede will give you a feel for the context of the time (700s-800s) and tell you a lot about how the early kingdoms developed before the Danes arrived and Alfred the Great unified what became England.

(Edit: for clarity)