The unilateral declaration of Scottish independence is the unlikely part. I am quite sure the Army would be willing to defend the union by committing the same sorts of acts they did in Ireland with the same level of enthusiasm.
I doubt there’d be too many people in Westminster willing to repeat the Troubles with a similar level of entusiasm.
I think it all depends. Perhaps few if any would want to kick off a violent conflict on purpose, but are plenty of clowns who would stumble into that situation through a series of poor decisions. If someone like the former PM Truss were in power, all bets are off.