Professor Garel Rhys of Cardiff University, writing of the Dark Ages, says that 5th and 6th century Britain was not dominated by Anglo Saxon culture (referring to a series of earlier letters). Instead, it was British Romano-Christian that was the most widespread culture and the religion aquired many new adherents prior to AD600, even in eastern Britain. The Germanics were not in a position before the mid 6th century to challenge British military and political dominance. Between AD570 and 630 much changed and the Anglo Saxon kingdoms (not a country called England) began to emerge. What the archaeology is really saying is that far from being the first part of the Roman Empire that was destroyed in the 5th century, it was in fact the only part of it in which the descendants of the Roman citizens lived under their own rule and this occurred over 200 years after AD400 (when the Roman armies departed). They had their own Roman Christian culture and late Roman-type political units. The British (Welsh and Cornish for example) were still a force to be reckoned with at Agincourt, in the medieval period.
This is the Dark Ages from a western British perspective, and it is interesting that this culture weakened after 630 – when the Arabs conquered a large part of the Mediterranean world, from Syria to Egypt and Libya. It is also significant, perhaps, that the decline is associated with 570, coming shortly after the 536-45AD narrow growth tree ring event and the plague that struck the country at about the same time (or as late as 550AD). It also appears to be the same events as described somewhat melodramatically by Gildas.