As British citizens we have the right to travel the length and breadth of our United Kingdom without let or hindrance. No devolved administration has the legal right to close the boundary between any part of the United Kingdom, including local authority areas, nor does it have any right to patrol these with any police presence whatsoever; without the express permission of the appropriate UK Secretary of State.
One of the most powerful arguments against separation is the idea that "we don't want border checkpoints between Scotland and England".
Since this is a powerful argument for Unionism, then it becomes the job of the SNP to attempt to normalise the idea of a policed "border" between Scotland and England.
If they can get people thinking that there is already "a border" which is being policed, and that it is perfectly natural for it to be patrolled in this way, then it will be much easier to move people towards accepting that there is nothing wrong with internal physical border checkpoints, and it is just another natural thing that we should accept.
This is what the SNP is trying to do right now. The Covid situation has handed it the perfect opportunity.
They want to establish a form of partition by the back door!
For example, a report in the Daily Telegraph (Daniel Sanderson, "Border checkpoints move a step closer", Scottish edition, 11-2-21, p. 7) claims that an SNP MSP describing himself as the Scottish Transport Minister "suggested his government (sic) was considering road-and-rail checks at the border with England to prevent Scots from trying to cheat the new curbs, if necessary."
They think they can do these things because "Transport" and "Health" are devolved matters.