Re nation: a nation is the socially constructed identity I was talking about. It can be mono- or multicultural, and people from other cultures may be integrated, it's all vibes-based depending on the nation. But one thing is clear, if you're born into a culture you are part of the culture, and so through a civic nationalist logic you are automatically part of that nation. Also note that nation does not equal state or country.
RE stone henge: that was ~6-4000 years ago, many thousand years before the Anglo-Saxons, so all we have to go on there is material culture, because it's prehistoric. As far as I can tell from a quick wikipedia read, a first part was built by neolithic farmers from a material culture associated with Anatolia, modern day Turkey. A second part may have been constructed by the Bell Beaker people (referring to material culture again) who arrived later. There was also an existing hunter-gatherer substrate before these other groups arrived. That substrate was mostly replaced by the neolithic farmer culture, which was in turn mostly replaced by the Bell Beaker culture. This replacement was a fairly long process, and the gene pool was also largely replaced during both of these transitions.
Both of these transitions are long before the Celts, Romans, Anglo-Saxons, etc., the labels we currently associate with Britain.
And the last point is my mistake, I thought you were the same guy as before, but your statement's premise was contradictory. You can't replace yourself, can you.