Why did Britain experience such a big decline anyway?
Allow me to cross-post from another thread, if you will, keeping in mind this is a rough summary.
Eventually it all comes down to the nature of roman rule in Britain. We know that romanisation was, rather than a systemic acculturation, a more or less deepened creolisation of provincial society based on political integration and an extensive trade and use of material culture (what we could call a "Roman-way-of-life"). Some people were latinized in the process (such as in Gaul), some were romanized along their own organised network and cultural structures (basically all the eastern Romania) and for some it was relatively more limited.
It was the case of Roman Britain that, up to the early IVth century, was essentially a military province for what mattered Rome with a significant urban/latifundar romanisation happening mosrly in the South (
and not everywhere in the South) while most of the roman structures in the province depending from military presence for exchanges, subsides, etc. Would have the empire fallen in the Third Century crisis, post-Roman Britain would have significantly less structures inherited from the Empire, being closer to Illyricum on this regard.
Still, things changed a bit after the IVth century because Brittonic society really began to romanize itself due to the need to compensate for the end of heavily militarized provincialism : more and more local products were exchanged, cities were less monumental but practically tought and well-maintained. Basically, a crisis managment that, so far, did work out without making Roman Britain sort of Roman Gaul expy, but his own things with pre-conquest structures (which never really disappeared, especially in peripheral regions) being integrated and integrating imperial feature.
It didn't last this long : Britain was targeted by neighbouring peoples such as Scotti or Pictii. These weren't newcomers but confederations of Gaelic and Brythonic peoples that appeared one century before, both to defend themselves, and to manage long-range raids. Let's say they weren't happy with the lesser ammount of their subsides so far regularily payed by the Empire and as
Rome withdrawed troops from the island to make up for the lack of manpower on the continent, well, they raided the heck out of the province.
Note that it's possible that you already had Saxons in southern Britain, as foedi or laeti in the Litus Saxonnorum (Saxon Shore). Robin Fleming disagrees, but I'm not really convinced by the arguments : it was common enough in the IIIrd/IVth century Gaul, so I could really see a Saxon coast guards against Saxon piracy, a bit like Normands in 911. Anyway.
It thus happened that Britto-Roman society was significantly weakened, and eventually Constantine III took with him the last regular troops, and Rome had no choice but to say "Well, you're on your own now.".
This being said, you certainly still had militias in Britain, probably with some comitatenses and more-than-token cavalry. But eventually, the only direct authority was gone, and you had a mosaic of municipal authorities, generals and capitains turned warlords, peasant communities and big landowners.
This alone was pretty much destructing but Britto-Roman society still had contact with the Roman state, notably by its presence in Vth century Gaul : Riotomagus (probably more of a title than a name, I'll come back to this) had an important strategical and military role in Northern Gaul.
But the collapse of the Roman state in the west was another taken shot : this fall was felt with particularly destructing
effects up to Scandinavia where appeared all the signs of geopolitical anarchy and renewed warfare. Britain basically lived trough two fall of Rome.
At this point, this much is clear archeologically, Britto-Roman society as I described is in ruins, not just trough raiding but by sheer exhaustion and relative inability of the elites to exchange with the continent. Still viable ruins, granted, but ruins nevertheless.
When Germans came "en masse" (relatively wise), they didn't as much ignored or fought Roman structures that they didn't found much of these.to being with : most first groups of migrants (from all the North Sea, most probably Saxons in majority,, but including Angles, Danes, Franks, Jutes, Norses, Frisii, etc.), or at best found them while they were collapsing. While some probably came as foedi, against Picts and Gaels, possibly against Armorican Saxons as well; most were coming as familial communities as Slavs did in the VIth century eastern Europe; not caring at the latest of collapsing imperial structures.
They mixed with remaining Britto-Roman structures and communities (Eastern Anglo-Saxon kingdoms essentially espoused early Britto-Roman entities), and took the lead thanks to better connections with the mainland : namely they were at the finish end of Channel/North Sea trade roads to Mediterranean Sea trough Gaul, which they did not only geographically but with remaining ties with kin communities in Gaul and Germania.
From there, relatively acephalic and tribal ensemble mixing up with relatively acephalic and territorial ensemble of eastern Britto-Romans, it allowed German leaders to take the presidence over economic matters and mobilising capacities; while Germans (especially in North-Eastern and Southern Britain) already had an edge as mercenaries or more militarized societies (due in no small part to the collapse of Northern Germanic ensembles with the fall of Rome).