I'm going to use Ireland as an example of what happens if you try that sort of thing without caution. In the beginning the majority of the Irish people would have been content with home rule. A bill to that effect was passed by Parliament in 1914, but the war got in the way of its implementation. Then a small group of republicans staged the Easter Rising, which was brutally crushed, and suddenly support for independence skyrocketed.
If Scotland ever faced a similar situation then the British government response would be nowhere near as brutal, for fear of repeating the mistakes of Ireland, but a blockade would still be an excellent piece of propaganda for Alex Salmond to use. I suppose the only long-term chance the union would have is for the Scottish people themselves to reject independence and ask for British intervention, unless we want to face international and domestic condemnation and risk permanently damaging Anglo-Scottish relations.