Reddit reviews Sutton Hoo and its Landscape: The Context of Monuments

There are a number of theories currently being floated, but I don't think we have a conpletely satisfying answer to your question (which is, I think, the most important question to ask). We assumed, for so long, that the 'Anglo-Saxons' claimed to be Angles and Saxons because they simply were, and we've struggled to adapt our understanding to the realization that these identities (like all 'national' identites) were inventive reimaginings of complicated history rather than the inevitable products of simple migrations of different, discrete people groups.

James Gerrard suggests that, after Roman power broke down in the fifth century, elites began to assume more personal responsibility for fighting off bandits and conteolling violence which, before, had been reserved (mostly) for the Roman army. As a result, elites started displaying weapons and furnishing bands of warriors about themselves - and these war bands were modeled on the 'barbarian' styles that had been popular in the late Roman army itself (the late Roman army dressed as barbarians, named its units after famous barbarian groups, and recruited heavily from barbarians, and so by the fifth century, the line between Roman solider and barbarian had grown very fuzzy). Gerrard's argument gets a bit fuzzy toward the end as he tries to explain why militarization resulted in the adoption of barbarian identities as well as barbarian military clothing and weapon styles, but it boils down to the idea that the old Roman institutions weren't a useful political model anymore, and claiming to be barbarians made more political sense when organizing small kingdoms with warbands at their core.

Toby Martin recently looked at the development of brooch styles in East Anglia, and shows how brooches suggest early connections between Anglia (in Germany) and East Anglia in England. He suggests that the movement of brooches among people in these regions is tied to the movements of other kinds of connections (not just migrations, but also friendships, trade, etc) across this north sea region. Tom Williamson made a similar argument a few years ago about the ease of travel between Anglia and north-east England. This suggests that the Angles were a good neighboring region to claim connections with, and graves like Sutton Hoo (which had a lot of goods from all around the north sea) show that having connections across that body of water were really important in the early seventh century. Claiming to be Roman (or Trojan) might sound very grand, but claiming to be related to the ancestors of important trading partners across the north sea was possibly more useful.

But we might also (and I'm sliding into speculation here) benefit from comparing changes in English identity to events on the continent. Britain might still have claimed to be Roman in the early sixth century (Gildas, a British preacher writing sometime in the first half of the sixth century, thought his people still had some kind of affinity for the Romans across the chanel), but Emperor Justinian's wars in the mid sixth century showed that the Roman empire did not consider the kingdoms ruling the terrorities of the western Roman empire to be legitimate successors to Roman identity: Theoderic may have claimed to be ruling Italy as a representative of the Roman empire, but Justinian saw the Ostrogoths as an enemy to be driven out of Italy. It's not very long after these events that we start to see evidence of the new Germanic identities emerging in England (and there are also changes in how the Franks in formerly Roman Gaul saw themselves in relation to the Roman empire). Perhaps, with the Roman empire flexing its muscles across the channel, it was safer not to base your political identity on anything too closely related to Roman history, and instead choose mythical ancestors who had successfully resisted Roman conquest back when the empire was at its height?