The end of Roman Rule.
In 406 AD the army in Britain elevated the first of three pretenders. There is some reason to suspect that the Roman Army was worried about a barbarian attack crossing the channel, for by now German invasions were sweeping through Gaul. There was little hope of getting help from the main Roman forces under Stilicho (Honorius' general) who was now plotting with Alaric and the Goths for an attack on the Eastern Roman Empire! The last of the three pretenders was Constantine III. He moved swiftly from Britain into Gaul and took over control of the surviving Roman Units there. By the end of 407 AD and after some stiff fighting (much of it against loyalist supporting the Emperor Honorius) he was repairing the Rhine defences. Despite attempts to get rid of him, Honorius was forced to recognize Constantine III as Augustus. However in 409 AD due to treachery within his own army as much as from external attacks, Constantine III territory started to fall apart.
The expulsion of the Roman officials
In 409 AD Britain revolted against Constantine III expelling all his officials. At the time people probably wouldn't have seen this as the end of Roman Britain; Britain had rebelled many times before and had always been brought back into the fold. However this may have been the point at which the Romano-British decided that they had had enough and wanted to go without the expensive trappings of the empire, whose administration costs would have been vast.
The end of the Roman Army in Britain
The Roman army had been very unpopular under the late Empire, it had proved politically unreliable often bringing disaster and civil war in its wake. At best it had been tolerated and at worst hated. The savage penalties for desertion or evading conscription suggest that service in it was something to be avoided and will help account of the increasing use of barbarian soldiers. Yet right to the end, Hadrian's wall was garrisoned and there would have been sizeable number of troops left in Britain.
While there is a great deal of uncertainty about what happened to the Roman Army in Britain; it's likely that it followed the same pattern as elsewhere in the Empire. With the split from the Empire the pay would have stopped arriving; when this happened on the continent some units disbanded themselves. In one case on the continent a unit sent delegations to try and find their pay. It was only when these failed to come back that the soldiers realised the truth and gave up. Some of the soldiers may have left to find more lucrative opportunities elsewhere, however others stayed around their old forts, farming the land or serving one of the new warlords.
What was left after all of this was not a centralised system based in one of the cities, but a totally fragmented one, with local arrangements changing from place to place, and an emerging number of petty warlords. One of the greatest surprises is how the use of coins almost disappears within 10 to 20 years. This shows the implosion that happened in Britain; so that by the mid 5th century it was in many ways a more backward country than the one the Roman had invaded four hundred years before.
In the end the Wall wasn't overrun by Barbarian Hordes, but instead became irrelevant, the garrisons still manning their posts, while the raiders attacked the vulnerable coasts to the South. Finally Rome was no longer able to pay the price for this imperial outpost.