I have no idea if Scottish Independence would be a good idea or not.

But i must say this is how a secession should happen, both from the secessionist side and from the central goverments. The secessionists used the normal channels (such as candidating for parliaments) to make their case. After they get some power in elections, the central goverment agrees to a referendum and the seccessionists and the central goverment find an agreement to the exact terms for the referendum. Both sides try to win the support of the population.

What i don't really understand is the anti secessionists argument, that they could prevent an independent Scotland from using pounds as currency. It has happened that countries use foreign currencies (IIRC Montenegro used D-Mark and later Euros). From the 70s to the introduction for the Euro, Austrias monetary policy had been to keep our Schillings at a fixed exchange rate with D-Marks, so we had de-facto D-Marks with a different name.

So if an independent Scotland either decides to use British Pounds as legal tender or introduces it's own currency, and has it's national bank follow the UKs monetary policy, what could the UK goverment do?

The most the UK government could really do in practice if Scotland uses the British pound is not help. Not supply currency, not act as a lender of last resort, press charges in international court if Scotland starts printing British pounds (which would be counterfeiting, after all), that sort of thing. But if they're crazy enough to make whatever the Bank of England thinks is good for England into Scottish policy? Then I suppose the English will let them...