Peter,
Given the amount of time between the Roman "official" departure to the influx of the germans I would think that they would have assimilated into the local culture through inter-marriage and land owning and might have been reluctant to join the influx of tribes.
Plus, I am not sure how much we know, right now, of the numbers of German auxiliaries in Britain at the beginning of the 5th century.
These are my guesses based on readings so far, so take them with a grain of salt, a loaf of bread and a jug of gaulish wine.
Felix Anno Novum