The UK can't refuse to have a currency union in practice. It might go as far as the European courts or other international bodies, but the currency is basically as much theirs as it is the rest of the UK's.
Actually the rUK can refuse a currency union, and it will - there is nothing stopping it, and no foreign court has jurisdiction here. There is nothing stopping Scotland from *using* Sterling as its currency, but that's not what's being discussed here as I said in my earlier post - Salmond wants a seat at the table when it comes to Sterling fiscal policy, while none of the Westminster parties want to sit next to him, because they do not want to be beholden to a second economy when setting fiscal policy for the rUK.
That's what this discussion is all about. Why should Westminster have to share fiscal decision making with an "independent" Scotland?
Go ahead and use Sterling as the thing you use to buy and sell things - but you aren't getting a seat at the Bank of England table.
Infact, Salmond could quite easily take the entirety of the Bank of England and Sterling with him, but that won't solve his issue - it doesn't get him a stable currency because the Bank of England will no longer be backed by the Westminster treasury, and as the Bank of Englands assets would still be sliced up as before, he wouldn't get any more money with which to base his lender of last resort on.
What Salmond is after when he says "I want a currency union" is actually "I want a backing lender that I can rely on to bail me out regardless, but I don't want to set up my own backing lender because that is costly and would mean I would have to renege on my taxation promises, and anyway said new backing lender would not have the standing on the international financial markets because of its lack of history and backing of an established economic policy and government treasury, so what I actually need is a backing lender linked to the Westminster government. Crap."
Problem is, the voters don't understand the complexities of all that and simply believe Salmond...
If the vote is yes then the rest of the UK will negotiate a union because it's in their best interests. Otherwise investors are going to start pulling out of the UK fast because if Scotland doesn't keep Sterling the rest of the UK's debt will increase massively in proportion. Sterling would also lose many of the assets it is valued against, like North Sea oil.
Investors aren't going to go anywhere, because the financial worth of the City of London far outweighs the potential revenue of the north sea oil - don't get me wrong, that oil revenue is a nice to have, but it won't break the rUK not to have it. The bulk of the GDP of the UK resides outside Scotland, so we aren't in anywhere near as much of a sticky place as you think we are, especially as most large Scottish financial institutes will have to move south of the border to satisfy EU and WTO regulations.
Sharing fiscal policy with a brand new government, one that has to find its legs, sort out internal taxation, find the balance of its people etc - thats not something we want as a country, because all that brings uncertainty and instability. Salmond can have all of that, we will just get on with our own fiscal responsibilities thanks.
I also don't see how the rUKs debt will "increase massively in proportion" - if Salmond tries to make good on his threat of not taking Scotlands portion of the national debt, then the fledgling Scottish treasury will have a fairly poor international credit rating, right at the time it needs to be borrowing in order to set up its central bank.
Don't fall into the trap of believing Salmond and his supporters when they link a currency union to debt - a countries debt is not linked to the currency that country uses, its an entirely separate thing. It might be denominated in that currency for the purposes of reporting, but it isn't linked to it - it doesn't magically go away if the UK stopped using Sterling, and it doesn't mean that Salmond can legitimately refuse to take the Scottish share of debt without a currency union.