Remember that the ancient Saxons (who would have most likely would have arrived via Denmark) were part of the British establishment by the 870 a.d. era depicted in the movie.
The Vikings raided Russia (or more precisely, the Baltic States) first not because they didn't know about the British Isles but because it represented a closer and softer target.
Yes, I was stupid and gave them a 2nd chance. I’m still mad at myself.
Going to Russia was much easier than going to Britain ~
They show up later as Vikings ~ but they were not terribly well educated when they left Europe ~ a real Dark Ages type people.
So, Freis who went West ended up in England (that's the 'en' in England, and those who went North suffered the punishments of hell with recurring famines, plagues, and all that. Their technological prowess declined substantially until they got the boat design that enabled them to RAID EUROPEANS ~ which is another couple of hundred years later.
The Vikings are a 'culture', not a genotype.