Based on the writings of the early 6th century historian Gildas, apparently there was some sort of functional post-Roman government in Britain with a council based on representatives of the different major cities/polities in the old Roman territories. Their downfall came after inviting Saxon settlers onto the island, with the leader of the government at the time (Vortigan?) being ironically among the first to die. Supposing the Saxons are not invited to settle, what sort of government evolves in Britain and how might this impact early Medieval and Dark Age Europe?