Continuing on for a moment on the topic of late Roman administration, I offer a citation from J.N.L. Myres articles 'Pelagius and the End of Roman Rule in Britain' (JRS 50  21-36). He notes at the opening of the article that '[t]he circumstance in which Roman rule over Britain came to an end have always been something of a puzzle to historians' (21). But he fills out the picture a little bit on p. 23:
The first decade of the fifth century, which saw the hey-day of Pelagius' influence and popularity in Rome, was marked by tremendous events in the political world, events which accelerated the disintegration of Roman authority in the West. Alaric and his Visigoths entered Italy in 402: in the last days of 406 vast hordes of barbarians, Vandlas, Alans, and Sueves, crossed the frozen Rhine and penetrated deeply into the Empire from which, in fact, they were never ejected again: almost at the same time came the revolt of Constantine III in Britain, which was quickly followed by the removal of the Roman armies and the ejection of the civil administration from this country: in 408 came the death of Stilicho, the last of Honorius' generals to continue the policy of the great Theodosius in dealing with barbarian penetration of the Empire: and in 410 Alaric sacked Rome and the government of Honorius officially acknowledged its inability to restore its authority in Britain. From that time, as Procopius puts it, the Romans never recovered Britain, which continued to be ruled by tyrants.
Zosimus, in Book 6 of his Historia Nova (translation here) comments on the period and the revolt of Constantine:
Constans was afterwards a second time sent into Spain, and took with him Justus as his general. Gerontius being dissatisfied at this, and having conciliated the favour of the soldiers in that quarter, incited the barbarians who were in Gallia Celtica to revolt against Constantine. Constantine being unable to withstand these, the greater part of his army being in Spain, the barbarians beyond the Rhine made such unbounded incursions over every province, as to reduce not only the Britons, but some of the Celtic nations also to the necessity of revolting from the empire, and living no longer under the Roman laws but as they themselves pleased. The Britons therefore took up arms, and incurred many dangerous enterprises for their own protection, until they had freed their cities from the barbarians who besieged them. In a |175 similiar manner, the whole of Armorica, with other provinces of Gaul, delivered themselves by the same means ; expelling the Roman magistrates or officers, and erecting a government, such as they pleased, of their own.
Thus happened this revolt or defection of Britain and the Celtic nations, when Constantine usurped the empire, by whose negligent government the barbarians were emboldened to commit such devastations.
Finally, for a some more general background on the upheavals occurring around 410, we can go to P. Brown's biography of Augustine (rev. ed., pp. 288-9):
Augustine was a bishop. His contact with the outside world was through pious Christians. He wished to 'weep with those who weep'; and he was genuinely annoyed that the Italian bishops had not troubled to inform him of the extent of the disaster. As an African bishop, however, he was himself fully preoccupied by events nearer home. The authorities in Carthage panicked at this time: to allay discontent they issued a hasty edict of toleration for the Donatists. This action dominated Augustine's life at the time of the sack of Rome. He was faced with a crisis of authority in his own town. Donatist violence had been renewed, and with it, a revival of religious 'segregation' among the Catholics: his own congregation had begun to ostracize Donatist converts. Augustine was partly responsible for this bad atmosphere. He had been constantly absent: he was still in Carthage on September 8th, 410, receiving urgent letters to return to Hippo. On his return, he was faced with far more pressing problems than the news of the distant sack of Rome: a converted Donatist had lapsed through being cold-shouldered by the Catholics. This is what really moved him: 'At that news, I tell you, brethren, my heart was broken: yes, my heart was broken.'
As a bishop, he had looked to Ravenna, where the Catholic Emperors issued the laws that protected his church, not to Rome. Thus, while Britain became independent, and Gaul fell to usurpers, Augustine and his colleagues remained loyal to the existing Emperor--Honorius. The father of this 'pale flower of the women's quarters', Theodosius the Great, will be presented as a model Christian prince in the City of God. There were good reasons for such a superficial panegyric: a law reaffirming all previous legislation suppressing non-Catholics had emerged from the chancery at Ravenna at almost exactly the same time as the Goths entered Rome.