What was going on on the British Isles in between the end of the Anglo Saxon’s and the time that Rome began to settle there?

The English, or Anglo-Saxons, arrived in Britannia at the tail end of the Roman occupation of the British Isles. The Romans had occupied Britain for nigh-on four centuries by the time that the final Roman military forces left the province in 410. Much of the North-Western garrison - that actually keeping Irish and Pictish raiders at bay - had actually been withdrawn in the 380s and had simply never been returned. The withdrawal of the Roman garrisons was part of a general collapse of centralised Roman authority and economic power, but one which crucially left the vast majority of the Romano-British population remaining. With the absence of Roman garrisons or authority, the new sub-Roman polities struggled with waves of raiding. Indeed, Roman legions purportedly returned to Britain on at least one occasion to disrupt a particularly sustained period of raiding that, according at least to Gildas, threatened to overwhelm Britannia entirely. It's likely during this period that sub-Roman polities, such as they were, began to fracture into increasingly smaller and more self-sustaining states, likely a resurgence in part of pre-Roman 'tribal' entities. Certainly this is a period which sees the reoccupation of a number of Iron Age hillforts and an increasing abandonment of less-defended urban centres, as well as evidence of potential Irish settlement in Western Cheshire and the Wirral. According to Gildas and Bede, these successor states did eventually manage to establish a measure of safety, stability and even prosperity, until a devastating plague swept through the former province in the late 820s, so deadly that 'the living could not bury the dead.' The knock-on effect of this was a vast resurgence in Irish and Pictish raiding.

The precise nature of the arrival of the English remains a matter of great historical debate. The model proposed by Gildas and Bede - that the English came as mercenaries who then betrayed the British and slaughtered them - has long been disproved; the English did indeed first arrive as foederati troops, but there remains much discussion as to the timing, scale and extent of their 'colonisation' or integration. The first English likely arrived in Kent, and indeed may even have been foederati there even before the Roman withdrawal in 410, but appear to have integrated themselves particularly thoroughly into the Sub-Roman state there; they identified themselves as Cantwara, the people of the land of Cantiaca - the Roman label for the area. In some areas, such as in St Albans, the incoming English appear to have carved out communities of their own, while in others, such as in Essex, archeological evidence suggests a settlement alongside, and then gradual integration with, existing Romano-British polities. Some areas, such as the Magonsaete in the south-west Midlands, may have seen barely any English presence at all, although may have simply adopted a Germanic material culture over time through trade and simple osmosis.