Constantine III was a Roman General who rebelled against the central imperial government and eventually took troops to attempt to seize control of the entire western empire. In order to do this he probably took most of the Roman field force of Britain with him. Adrian Goldsworthy makes the point that probably few of these troops returned to the province afterwards. (341) After local British leaders rebelled against Constantine III (who having been made Augustus in Britain had promptly taken his troops to Gaul leaving the British to deal with their problems themselves, the very problems that had pushed them into revolution. . .) Britain basically drifted away from the Roman Empire even if the citizens and towns still considered themselves Romans. The Western Imperial Government at Ravenna essentially had more pressing problems then attempting to reassert direct imperial authority in a distant province that had a long history of producing usurpers that would attack the central government. In this time frame there was the rebellion of Constantine III and then invasions of Gaul and Italy itself by the Visigoths. Another tribal group eventually even penetrated into Spain. The military and economic resources of the Imperial generalissimio Stilicho and his successors were hardpressed to defend core western provinces. There simply was nothing to spare for restoring a new garrison to Britain or reorganizing a new provincial administration. So by 411 AD (the death of Constantine III), or 407-8 AD (when Constantine III's government was expelled from Britain) can be given as a date for the Roman government abandonment of Britain during a massive military crisis in the early decades of the fourth century.

"The Fall of the West: The Death of the Roman Superpower." By Adrian Goldsworthy. 2009 Pages 335-352.

Edit: Bad sentence. Never see it until after its posted. :p