In the show 'Vikings', Ragnar and his warband end up landing at Lindisfarne in modern day England and promptly slaughter the monks living there.
In later seasons you are introduced to the English Kingdoms of Wessex, Mercia and Northumbria. My question is, is that we know that the Normans from Northern France in Medieval Europe are the descendants of the Vikings who settled there from around the same time as the Vikings show takes place and that the British isles during the time of Roman occupation was occupied by Picts and Celts. But who were the Wessex, Mercia and Northumbrian peoples descended from. They aren't Vikings or other Norsemen or Romans or Picts or Celts (as far as I am aware of) so who are they and where did they come from?
There is always more to be said, but you may be interested in these previous answers written by /u/alriclofgar:
Were the Anglo-Saxon invasions a catastrophic event that changed the face of England, or were they fairly insignificant outside of the effect on the language? (This one also answered by /u/haakon_vii)
The Kingdoms of Wessex, Mercia and Northumbria are what we today call Anglo-Saxon kingdoms. Before the Viking conquest, and later the Norman invasion in 1066, Britain began experiencing an influx of Germanic migrants into the region we roughly associate with England from the mid fifth century. These groups included the Angles, Jutes, and Saxons, who displaced and absorbed the Romano-Britons and became the dominant culture in those areas.
By the time of the 'Viking Age', a period which the show jumbles up considerably, these Anglo-Saxons had been long established in what is now England and included the Kingdoms of Wessex, Mercia and Northumbria, although there were other Anglo-Saxon kingdoms at various points (Kent, Essex etc).
So yes, they were not culturally Romans or Britons, while the Picts/Scots occupied their northern border and Celtic or Romano-Briton culture continued in Wales and Cornwall.
Several historical accounts exist that can relate to you the overall story of the Anglo-Saxon invasion/migration, although they are all writing long after the fact and each have their own bias and motives, so you should treat a lot of details with skepticism.
Ancient Sources (translations of these are easy to find for free online):
Gilda's 'De Excidio et Conquestu Britanniae'
Bede's 'Historia ecclesiastica gentis Anglorum'
'Anglo-Saxon Chronicle' (although portions of this text may have been written 600 years after the events)
This topic is well researched, so good books by modern academics are pretty easy to find and can provide a far greater depth of historical knowledge than merely the above ancient sources.