My answer here discusses this. My opinion is that no educated Anglo-Saxons would seriously believe this. The aristocracy, though perhaps less educated than monks, were still members of a Christian network of faith/patronage/personal alliances linked with Continental Europe, and so would have been fully aware of their Roman past.

I haven’t seen the show, but it sounds like they’re referencing the Anglo-Saxon poem “The Ruin”, which does indeed attribute Roman ruins (probably the city of Bath) to giants. The giants may be poetic license, but the rest of the fragmentary poem imagines the Roman ruins as having been mead-halls and treasuries for bands of warriors.

Shlin28's answer is perfect.

The Viking show is certainly great to watch, and they do some things fine. I especially like their portrayal of a non christian heritage in the way the Vikings speak and think. They fudge Ragnars response to things like murder and rape to make him palatable for western audiences, but other than that they dont do a bad job, considering. Although I'm sure purists will dislike a fair bit of it.

In other places they do a woeful job, mainly to show cookie cutter Vikingness at its best or as a plot device.

Even people early in the viking period, Ragnar is around the 800AD mark, were far more aware of the outside world than is portrayed in the show.

They make a big fuss in series 1 about finding the mysterious western land of England. However 200 years earlier their direct southern neighbours had no problems sailing across and occupying the place without magical lodestones. I'm sure fisherman from both sides would have met one another out at places like the Dogger Bank and that some degree of trade went on.

The English kingdoms, with their either fat crude northern king, or slightly effete cunning conniving southern king, are show as simplistic barely 200 people towns with several outlying villages. And as you say, with little or no understanding of the outside wolrd (well why would they, sitting out in the Atlantic in their lost island). As shlin28 shows though they had a clear understanding of the world and had extensive connections to it via trade, religion and diplomacy. I think we over simplify their world and assume that because little was recorded at that time that that means little was known or done.

Take anything you see in the next series with a grain of salt, there are some gross oversimplifications going on.

I'm not sure if this is too tangential to ask as a follow-up question, but at one point in season two a crowd in Wessex breaks into cheers of "Hail England" or something similar.

Would inhabitants of the English kingdoms see themselves as English or wessexian or northumbrian, as the case might be? Was there a concept of England as a nation around 800 AD?