The Byzantine historian Procopius (500-565), famous for his Secret Histories, writes about Britain in his seminal History of the Wars, although, frankly, he's all over the place.

Procopius likely gets most of his information from a Frankish embassy to Justinian in 533, which purportedly contains Anglian ambassadors, and a lot of his information is extremely confusing. To start with, Procopius talks about two separate entities: Brittia and Britannia, with very little distinction as beyond Britannia being 'to the West' of Brittia. From an assertion that Brittia was split in two by a wall, we can tell he means Great Britain, but then the location of his 'Britannia' remains somewhat confused.

According to History of the Wars 8.20.6-10:

Three very populous nations inhabit the Island of Brittia, and one king is set over each of them. And the names of these nations are Angles, Frisians, and Britons who have the same name as the island. So great apparently is the multitude of these peoples that every year in large groups they migrate from there with their women and children and go to the Franks. And they [the Franks] are settling them in what seems to be the more desolate part of their land, and as a result of this they say they are gaining possession of the island

If this information had come from Anglian ambassadors, it might explain why the Saxons and Jutes don't get a mention, but the Frisians do. The settling of Armorica and its transition into Brittany during this period is clearly reflected in Procopius' information, and may reflect events following the Battle of Badon. That Armorica is clearly referenced, however, puts paid to a theory that his 'Britannia' was a proto-Britany. Defining Britannia as being '4000 stades' off the coast of Spain, it is of course possible that he is conflating navigational information from both the Goths in Spain and the Franks and Angles to accidentally arrive at two Britains in the same place. The situation was almost certainly complicated by the potentially double language barrier between the Anglian ambassadors and Procopius, if indeed he was able to speak to them in person. This is also perhaps the cause of some of the more outlandish information, including this gem about Scotland:

8.20.42-8 Now in this island of Britain the men of ancient times built a long wall, cutting off a large part of it; and the climate and the soil and everything else is not alike on the two sides of it.  For to the south of the wall there is a salubrious air, changing with the seasons, being moderately warm in summer and cool in winter. But on the north side everything is the reverse of this, so that it is actually impossible for a man to survive there even a half-hour, but countless snakes and serpents and every other kind of wild creature occupy this area as their own.  And, strangest of all, the inhabitants say that if a man crosses this wall and goes to the other side, he dies straightway. They say, then, that the souls of men who die are always conveyed to this place.