OK, it may be a little difficult to find a suitable POD but I really think most of you are not giving this a chance. What if all the Scots in the Canadas and Maritimes unite with the homeland? Then you would have the Union surrounded and the English would have trouble supressing the rebellion as they would have to deal with disent at home and abroad.
I'm going to try to explain why most people are dismissing this idea, as you present it, out of hand.
Find a globe or atlas, or really any sort of map. Next, find scotland. Look to the south. do you see anything else on the island? If you do, that is England, a larger, richer, more powerful component part of the united kingdom. if you can't, then some glasses may be in order.
Next, look to the southwest. Find the confederate states of America. Notice how far away it is from Scotland. If you wish, you can calculate the distance from, say, Edinburgh to Richmond. Also, take note of the big country to the north of the southern states. Notice how very close the United States are, and how very far away Scotland is.
Do you see that blue thing seperating the American nations from Scotland? That is the Atlantic Ocean. The atlantic ocean is the home to a force of nature commonly referred to as the Royal Navy. Recall that, at this time, the Royal Navy was supreme on the world's seas. Think about what impact the now enraged royal navy could have on a united CSA/Scotland.
*cue
Rule Britannia*
The next step. go on google, or wikipedia, or really any vaguely reputable resource. Find some shred of evidence of an active, influential Scottish nationalist movement that could possibly declare independence without being laughed into the firth of forth.
Next, ask yourself two questions. What does Scotland gain by declaring independence? What could possibly motivate them to unite with the Confederates, of all people? Think about these for a minute, or an hour, or however long you wish. Do the answers seem logical?
Since you brought up Canada, add to these questions why Canada is getting involved with all these secessions and unions and such.
I just want to go on the record and say that one of my pet peeves with this board in recent times has been the tendency to simply reject ideas as ASB almost as an instinctive reaction. But, on the other hand, there are some cases where that moniker is entirely deserved. I'm not saying that your scenario is entirely lacking merit; with some modifications, it does raise a few interesting questions, some of which are already being discussed. But the concept that you are proposing makes absolutely no sense, for a multitude of reasons, most of which have been touched on already. If you can come up with good, logical, substantive reasons why we should reconsider the possibility of these events passing as you describe, then do so. But I can't come up with any.