I appreciate your point, but I don’t entirely agree.
The term “British” originated within the context of the British Empire, where anyone born in territories under the Crown’s control was classified as a “British subject.” Historically, “British” served as an umbrella term encompassing all subjects of the empire, from the British Isles to distant colonies. This usage has, of course, evolved over time. Nevertheless, it was always a broad legal designation for those under the dominion of the British Crown, rather than a precise marker of identity or ethnicity.
In your example of individuals considered “British” with family ties to anglophone former colonies or current Commonwealth nations—yes, they might be ethnically English, Scottish, Welsh, Irish, Cornish, or Manx, but ethnicity alone doesn’t define “Britishness.” “British” has never been an ethnicity; it’s a political and legal construct tied to subjecthood, and to some extent a shared cultural heritage.
As for the thought experiment I proposed, I described it as imperfect; however, I don’t believe your example undermines it. If someone has a mechanism to escape this land—meaning they aren’t absolutely and totally dependent on its survival, nor hold it as the core of their identity, home, and future—they are, in my view, less “British” than someone who does. To me, “Britishness” now implies a deeper, more intrinsic connection to these specific islands and their fate, beyond mere legal status or ethnic background.
What’s more, tying “British” to the ethnicities of these islands, I believe, diminishes the significance of the distinct heritages tied to them. Those identities—English, Scottish, Welsh, and so on—are far more definitive than “British,” which remains a nebulous and imprecise term. That’s my view.