Constantine III – Gibbon’s Decline and Fall of the Roman Empire Chapter 30 Part 3
The troubles in Italy and Gaul left Britain isolated. Its imperial forces had been withdrawn leaving low level garrisons to defend it from the depredations of the Picts and the seaborne Saxons. Being right at the bottom of the imperial todo list created a power vacuum in the province. There was a long tradition in Britain of providing usurpers to the throne, so it was sort of inevitable that a pretender to the purple should emerge.