If the Scots choose independence, they could be forced to put up border checks along the English border due to EU rules.
As always with the EU, it’s all very ambiguous and technical and depends on whether Scotland rejoins from within (that would be Article 48) or has to leave and reapply (you’d use Article 49), and that’s where keeping the border just as it is, except for painting in the words “Alex Salmond’s fantastic new, improved, independent” between “Welcome to” and “Scotland” on the sign, comes unstuck.
If Scotland reapplies, no matter how much that’s a technicality, current EU rules say that it would also have to sign up to the Schengen travel area, which allows people to travel freely across international borders with minimal checks.
But the rest of the UK has an opt-out from Schengen, which would mean Scotland would have to set up full border checks with England.
This would be hugely damaging not only for business, and people travelling from Scotland to England or Northern Ireland, or vice-versa, but also a huge problem for the Border Force of what would become the rUK. It would also increase the pressure on the rUK and Eire join Schengen.
Scots who live near the border will be especially affected. They cross it all the time because their nearest big town is often on the other side; and there’s a lot of cross-border business. One honey producer 200 yards into England has bees on both sides of the border.
The same kind of traffic exists between Northern Ireland and Eire, which is why the whole of the UK and the Republic of Ireland have a Common Travel Area (CTA), and there are in effect no border checks or documents required within their borders.
Even if there were a divorce which turned acrimonious, or if the taxes on some goods (petrol, ciggies, whisky) were lower on one side, it probably wouldn’t be in anyone’s interests to change this set-up.
Besides, Hadrian’s Wall is a listed monument, and it would be a bugger to get the planning permission to turn it into Checkpoint Charlie, even if it wouldn’t also need moving a few miles north.