The Britons were the original inhabitants of historical England, as early as 10,000 BC they were nomadic tribes living in particular areas, living on natural resources of its territory.
One of the first descriptions of the Britons was by Ceasar, when he visited the island.
In 410 A.D. when the Roman Legions were recalled to Rome to fight off the barbarian attacks, the Anglo-Saxons started to arrive.
Interesting links:
History of the Britons by Nennius
After the Flood: The Chronicles of the early Britons