This is not my work, it comes from ‘Rail Forums UK’ – the best explanation of citizenship post indyref I’ve seen anywhere.
Rail forums UK
‘So you think to yourself – Of course the rUK will want to keep out the Scots who will now be foreigners in rUK with no right to be there?
Unfortunately for this argument, citizens of iScotland born before 2017 will have an automatic right to British nationality as a result of having British nationality acquired before independence.
Many may choose to take dual nationality at the point of independence (as I certainly would).
Going forward Scottish citizens would continue to have freedom of movement to the rUK as part of the EU so there would be no economic/political benefit from preventing iScotland citizens from moving freely over the border.
The only people you are concerned with are non iScotland citizens entering iScotland externally and then crossing the open border.
This is exactly the same as the Irish situation today and the reason why visa harmonisation between rUK, ROI and iScotland would continue post independence.
Now again you may say one/both countries could leave the EU in future. This would change matters slightly but not that much.
All iScotland citizens born pre 2017 would continue to have the right to live and work in rUK via dual nationality, or if they haven’t taken this status they would have a similar right to the Irish citizens right of abode in the British Nationality Act 1981.
Additionally the children and grandchildren of people born in the UK before 2017 would automatically qualify for rUK UK Ancestry Entry Clearance giving the right to work in the rUK for 5 years and then acquire indefinite leave to remain/rUK citizenship after that point (this is the method used by all the Aussie backpackers).
The only people living in iScotland who wouldn’t have the right to work in rUK, even if rUK left the EU, would be immigrants to iScotland who took out citizenship after 2017 and any children they had who did not have a single UK born grandparent.
So until 3 generations have passed from 2017 and rUK has left the EU and the EEA and has no bilateral freedom of movement treaty there will be negligible numbers of iScotland residents who do not have a legal route to work / live in rUK.
Therefore the economic benefits of a border (keeping out illegal immigrants) would be very low as there will be almost no one living in Scotland who could be an illegal immigrant to rUK until at least 2070 or so.’