Start by discussing things in the correct language. The last time Cornwall was a separate entity, what we call England was split up into a number of kingdoms, with each king barely ruling what we would now term a county. The Union was a union between two states, England and Scotland. By that point, Cornwall had not been an independent entity for a thousand years. Cornwall only has a separate geographical status for administrative reasons. As a realm, which predates what you might recognise as a state, it is inseparable from "England". Start by recognising that not all the world fits into American conceptions of states, laws and rights.
Do we talk about the legal status of Islington North within the Union? No, because Islington North is part of England, and only exists as an entity for administrative reasons.
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