On Britain and Brittia
From Procopius' De Bellis
c. 540s CE
3.2.31-39
[Years 408-450] And the island of Britain revolted from the Romans, and the soldiers there chose as their king Constantinus, a man of no mean station. And he straightway gathered a fleet of ships and a formidable army and invaded both Spain and Gaul with a great force, thinking to enslave these countries. But Honorius was holding ships in readiness and waiting to see what would happen in Libya, in order that, if those sent by Attalus were repulsed, he might himself sail for Libya and keep some portion of his own kingdom, while if matters there should go against him, he might reach Theodosius and remain with him. For Arcadius had already died long before, and his son Theodosius, still a very young child, held the power of the East. But while Honorius was thus anxiously awaiting the outcome of these events and tossed amid the billows of uncertain fortune, it so chanced that some wonderful pieces of good fortune befell him. For God is accustomed to succour those who are neither clever nor able to devise anything of themselves, and to lend them assistance, if they be not wicked, when they are in the last extremity of despair ; such a thing, indeed, befell this emperor. For it was suddenly reported from Libya that the commanders of Attalus had been destroyed, and that a host of ships was at hand from Byzantium with a very great number of soldiers who had come to assist him, though he had not expected them, and that Alaric, having quarrelled with Attalus, had stripped him of the emperor's garb and was now keeping him under guard in the position of a private citizen. And afterwards Alaric died of disease, and the army of the Visigoths under the leadership of Adaulphus proceeded into Gaul, and Constantinus, defeated in [411 a.d. ] battle, died with his sons. However the Romans never succeeded in recovering Britain, but it remained from that time on under tyrants. And the Goths, after making the crossing of the Ister, at first occupied Pannonia, but afterwards, since the emperor gave them the right, they inhabited the country of Thrace. And after spending no great time there they conquered the West. But this will be told in the narrative concerning the Goths.
From Book VIII: On the Isle of Brittia:
"They imagine that the souls of the dead are transported to that island. On the coast of the continent there dwell under Frankish sovereignty, but hitherto exempt from all taxation, fishers and farmers, whose duty it is to ferry the souls over. This duty they take in turn. Those to whom it falls on any night, go to bed at dusk; at midnight they hear a knocking at their door, and muffled voices calling. Immediately they rise, go to the shore, and there see empty boats, not their own but strange ones, they go on board and seize the oars. When the boat is under way, they perceive that she is laden choke-full, with her gunwales hardly a finger's breadth above water. Yet they see no one, and in an hour's time they touch land, which one of their own craft would take a day and a night to do. Arrived at Brittia, the boat speedily unloads, and becomes so light that she only dips her keel in the wave. Neither on the voyage nor at landing do they see any one, but they hear a voice loudly asking each one his name and country. Women that have crossed give their husbands' names."
Notes
I include the section on the mythical island of Brittia, which Procopius says lays between the isles of Brettiana and Thule, as it is curious for two reasons: first, because the island's name is so similar to that of Britain, and inhabited by Angles, Frisians, and Britons, but is explicitly not Britain. Meanwhile, it is an island of the dead, not unlike the Irish island of Tech Duinn, where Donn the first Irishman presides over the dead, as recounted in the Dindsenchus.
SOURCES
Procopius. De Bellis. History of the Vandal Wars, Books III and IV (of 8). translator: H. B. Dewin. 1916.
On Brittia: from Wikipedia, as I couldn't find another translation. URL: http://en.wikipedia.org/wiki/Brittia.