I think those British in the Raj had British passports, though, didn't hey?You asked me a question and I gave you an answer, that.....as with happened in India with the British who had been there for generations....it's the country of linage.
The British seem to have been far more honourable about it.
How it happens? This has happened many times, I should imagine some arrangement is made with the original country.
People without allegiance shouldn't have a problem with leaving the country. They don't belong here and should be with the country they have loyalty to.
'Some arrangement'. Supposing their 'lineal' country refuses to accept someone as a citizen because they have a British passport?
As has been asked: how is 'allegiance' measured? And how would we know if someone is telling the truth?:
'In a cricket match, who do you support?' 'Well, England, of course'
'Do you have an allegiance to the Union flag or the flag of your parents' birth county?' 'England, of course'.
Easy.