Rise of the New Empire
The britons inhabit all of the roman-controlled territory in Britannia—that being all of the mainland south of Hadrian's Wall—as well as the territory south and a bit north of of the Antonine Wall. They were around before the Romans settled on the island, and their Celtic culture remains mainly intact on the west half of the island, where the Romans hadn't settled as much. They speak Common Brittonic, a Celtic language similar to Pictish, and many of them also speak Latin, though more commonly on the east side of the island.
Northern Brittania (Not part of the Western Roman Empire):