I am posting this question in Academia, as no other forums would seem to cover the area.
The Romans would have created many colonies in Britain, at strategic locations, where discharged soldiers settled after their enlistment. Boudicca took care to destroy the colony at Colchester, for example. It is logical that these people would have had families and communities. In other provinces, such as Gaul and Spain, they would have contributed greatly to the spread of the Latin language. However no such affect appears to have occurred in Britain.
When the Romans left, surely these communities, which would have been there for hundreds of years, would have continued on. Why did these communities, strategically located and descended from fighting men, simply fold when the boat people turned up? Compare to Dacia, where the Romans had colonies for a much shorter period, and where Latin most probably continued on to Romanian today.
My own way-out theory is that these colonies may not have spoken Latin. What if they spoke German? Could the Romans have simply settled all their German veterans in Britain? When the Romans left, they would naturally have allied with Germanic peoples against the Celtic British.
