@Lockdownbear
Waxon that's partly what I was thinking.
But then what's the motivation for them to allow Scottish frate (spelling?) to travel through England adding to their pot holes without charging some sort of road toll /tax?
freight

because we'd be allowing their freight up using our roads etc. I think in the end it would be 6 and 2 threes and it would mainly balance out. If we were using their ports etc to manage exports, i would imagine there would be a charge at the point of use for types of services that would be a cost with no benefit, if there was an imbalance.
With they way things are all tracked and electronically tagged, I don't think it would be too difficult to assess major business. I don't think either side would be too bothered about internal tourism or personal shopping across the border as it would work both ways.
Not that I'm trying to sell independence! :o
I think (taking immigration aside) that the border would be the least of our concerns.