The year 410 is the one traditionally given for the Romans leaving Britannia. We cannot be sure, of course.
With Italy itself under attack by Goths and other Germanic tribes, the Legions were gradually withdrawn from the outlying parts of the Empire. This left Exeter and other cities vulnerable to attack by Saxons, Irish and others.
Britain was entering a new, sub-Roman period, with many Roman influences remaining. This was the time that legend puts King Arthur just west of us in Tintagel.
The painting is ‘Romans Leaving Britain’ by the Victorian painter John Everett Millais