Whilst you are right that initially Britania continued as under the Romans it did not remain that way up until the formation of the Anglo-Saxon kingdoms. We know large provincial authorities fracture at some point. It is unusual for this to happen peacefully & having independent regions the size of towns almost always leads to struggles with raids, local resource conflicts etc.

We know the wall stops becoming defense & Celtic raids become more frequent with places like Vindolanda - which carried on as a civilian bathhouse after the troops left, was abandoned. We also can see the coin deposits found that were buried & never returned to.

Whilst it's true large towns & cities, the best places for finding evidence of violence in archeological record would have been largely insulated from this. It doesn't mean the countryside, where wooden buildings leave little record isn't being impacted.

So yes, Britain carries on as before when the military forces leave & do not return but we see a slow gradual decline of authority to local struggles towards the end of the next century.