I have actually researched a bit into British Law, and how it was they came to assert this idea that anyone born in England was automatically a British Subject.
As near as I have been able to tell it tracks back to James I and Calvin's Case.
There appears to be pretty good evidence that they dreamed the whole thing up to solve what had become a serious political problem. Had they decided the case the other way, Scotland would have split from the United Kingdom, and so the King very much needed Calvin's case to be ruled the way it was.
Even so, it took 14 judges over a year to decide that someone born in England was a "natural born subject" and they still had two dissenting judges.
I dare say they simply gave the King the ruling he absolutely had to have.
Just thought you should know. :-)