At https://phys.org/print419839930.html … archaeologist Susan Oosthuizen is well known for her research into the historicity of Britain's common land – which she claims goes back at least 1500 years. She was quoted by Steve Mitchell in one of his articles in SIS Review and has popped up in conversation with him, at one time or another. She specialises in the Anglo Saxon period and in particular, Cambridgeshire and the Fen country. The Fens were flooded in the Late Roman period – between 300 and 600AD, following the dry Roman Warm Period. This is what Steve Mitchell was writing about. The Fens are thought to have been a perfect water highway from the North Sea to lowland England – via the River Ouse and its tributaries, bringing hordes of Anglo Saxon migrants. Historians have argued that after around 400AD settlements in the Fen country were mostly abandoned and the region became a watery wilderness, virtually uninhabited. The area had been well settled in the Roman empire period and therefore this amounts to a dramatic decline – not just in economic activity such as farming but also in population numbers. Susan Oosthuizen is a landscape archaeologist with a number of books and scholarly articles to her credit. Her latest offering is 'The Anglo Saxon Fenland', Windgather Press:2017. She suggests that rather than undergoing dramatic change after 400AD communities continued to live around the Fen edge and on islands of higher ground in the Fens themselves, just as their ancestors had done. The introduction of the Portable Antiquities Scheme in 1997 has led to the public reporting many finds that would otherwise have remained unknown by archaeologists, particularly the activity of metal detectorists. Since the 19th century it has been assumed that during the 5th and 6th centuries indigneous communities in lowland England were removed altogether or reduced to servitude by Anglo Saxon incomers. There is now a growing realisation among archaeologists that it is impossible to differentiate, from material culture, Anglo Saxons from indigenous people. Very little actual migration seems to have occurred – although the arrival of warlords and mercenaries from Denmark and NW Germany appear to have married into the local aristocracy and in the process, became the dominant faction amongst the ruling elite. This applies to the Fens as much as elsewhere in lowland England. The western side of the country was less affected by migration from the continent and retained their Celtic culture and language. See also www.oxbowbooks.com/oxbow/the-anglo-saxon-fenland.html
This brings us around to evidence of newcomers at a different moment in history, the Beaker Folk who arrived in the second half of the 3rd millennium BC (about 3000 years prior to the Anglo Saxon period). A paper came out recently saying that the Beaker Period represented the arrival of large numbers of people with an origin in Frisia (in the Low Countries). Interestingly, this period also experienced a marine transgression and lakes and lochs expanded in Ireland as well as Scotland and England. Frisia, on the opposite side of the North Sea from East Anglia, was as affected by rising sea levels and water as much as Britain, and it is no accident this period coincides with the Early Bronze Age site destructions common to the Levant, Anatolia, and the central Mediterranean. Ignoring the actual trigger of what appears to have been a global catastrophic event it is remarkable that Beaker material culture pops up across most of western Europe, from Iberia to Poland. In Britain it is accompanied by an actual migration from Frisia which is interesting as the Frisian tongue is recognised as having close similarities in common with English language. Hence, one may speculate, supported by genetic evidence, the arrival of English speakers in lowland eastern Britain occurred as early as 2500 to 2000BC. This implies Bronze Age and Iron Age lowland England was not strictly Celtic as historians have assumed – but proto-English. It also represents another migration of the kind described by Moe Mandelkehr in his series of article in SIS Review occurring in the wake of a catastrophic event he dated to 2300BC (represented in tree rings at 2345BC). In other words, the Anglo Saxons of the post-Roman period spoke a Scandinavian tongue (preserved in the Beowulf poem) but the indigenous spoke proto-English (which was quite different). It makes sense to place the origins of English place names at the beginning of the British Bronze Age as many of the settlements and farms established at that time, and the road system, remained in continuous use, at the same place or nearby, all the way down into the post-Roman era. Landscape archaeology and genetics have revolutionised the history of the past in Britain which is timely in a way as many of the ideas of a mass A/S invasion are based on assumption rather than field evidence.