The Roman’s built these defenses because the Celts kept kicking their butts
The Romans tucked their tails between their legs and, oh, wait, they ruled Britain for another 349 years (AD 61 to 410).
The Romans built forts all around the Empire; the one known serious uprising in Britain was this Boudiccan revolt, during a period the legions were understrength in Britain due to a problem on a frontier on the continent, and the result of which was the slaughter of a reported 80,000 Celts by about a legion and a half of Roman troops doing what they did best in their most disciplined manner.
The Iceni ceased to exist.
Boudicca poisoned herself.