After 400, the economy of England reverted into subsistence agriculture, meaning there was little surplus food or wealth to put into building forts and earthworks—or even for sustaining cities. Stories that survive from this period—such as the Fight at Finnsburg—suggest that 'forts' were little more than a longhouse where a petty king could host a few dozen warriors. Archaeologists have found a few such sites, including one at Lyminge, but it's certainly not enough to base a map off of.

So-called landscape archaeologists, who look at the bigger picture rather than at specific sites, make a compelling argument that early Anglo-Saxon societies were held together by waterways, which make for easy transport in a world without governments to maintain roads. It's reasonable to assume that rivers and bays were often the centers of regions or micro-regions, rather than boundaries, although that doesn't necessarily tell us much about the particular places that chieftains might place their main estates.

Later on, beginning around 700, religious centers became the main points of authority. Royal families linked themselves closely with monasteries and—uniquely in England—double monasteries, where male monks and female nuns lived adjacent to each other separated only by a church. We know some of these sites like Repton because they're still used for church purposes today, others like Portmahomack we've identified through archaeological remains, and others like Clovesho are mentioned in documents although we have no idea where they were actually located. If you want to go in-depth, one point of access might be the Prosopography of Anglo-Saxon England database. You might also get a good idea of these locations by looking for where Anglo-Saxon bishops lived.

Archaeologists have also identified numerous sites that they call wics or emporia (singular, emporium). These were trading sites, and there was typically one per kingdom. They began popping up around 700, at Ipswich, Hamwic (Southampton), and London. I'm not much of an expert after that point, but if you have access to a library, you can probably get to this article, and even if you don't, you can probably pull a few things out of this dissertation.

Finally, the kings of Wessex gradually organized their power by establishing burhs as fortified garrison points. We have a list called the Burgal Hidage which lists tburhs, mostly in Wessex, slightly after the period you're interested in. It's reasonable to assume that many of these were already in use as defensive points before this time, although perhaps not formally identified as such. There's a helpful map of these locations here.

At any rate, the evidence is patchy, so it's no surprise you didn't find much. For the 400s and 500s, your best bet is to think about small river valleys as the units of political power, and there would probably be one or two longhouses where a 'king' would host his warriors on a more or less permanent basis. In the 600s, monasteries and abbeys start to pop up as new centers of power, although these wouldn't have been fortified. In the early 700s, towns called wics or emporia start to appear, but these were typically small and either unfortified or, in the case of London, located near old Roman walls. In the late 700s, there was a major earthwork project known as Offa's Dyke. And in the late 800s, the burghal system came into being, best represented by the Burghal Hidage. At the same time, navies also became important, although these are better thought of as troop transport fleets, rather than as warships meant to fight on the high seas.