Regarding the Saxon arrivals in Britain, St. Gildas gives us the dates ~ he was there when King Ad gave up the family farm (then Northern Cornwall, now Wessex/Essex/Susses/etceterex) to the Saxons ~ who we now know weren't all that numerous. The residents relocated to a recently depopulated Brittany.
The depopulation occurs after the end of the world event ~ and involves Merlin et al replanting grapes from there to the Rhone Valley.
That would be about 535AD give a couple of decades.
At the same time the Angles were still waiting in Freisland ~ their relocation to Britain happened later, and they began moving into a quite miserable Scandinavia and West towards Nederland and Britain.
The Vikings arise several centuries later in Scandinavia after they'd overextended themselves and were starving to death. Life was very difficult until they got their seagoing boats ~ in the 800s. By then their society had deteriorated into a dog eat dog primitive barbarism ~ with reading and writing.
Most people of Scandinavian ancestry don't like to think of it that way but that's the way it was.
It was 1000 AD before the missionaries got to Iceland to convert those people to Christianity.
To which I might add that it was global warming or climate change which facilitated the settlement of Iceland, Greenland and even (for a short time) the northeastern coasts of North America.