They can use any currency they choose. Whether they actually stay in the EU or not is actually irrelevant, they will see this as England forcing something on them against their wishes and the issue will be 'Who governs Scotland?' Of course the UK hasn't broken up yet but in my view it is more likely than not. Much will depend on what happens in our negotiations with the EU next year or so and what I would expect is that referendum enabling legislation will be passed in Scotland with no date set immediately. The threat will be left hanging. Whatever, the whole debacle has been very divisive and the Union is now looking a lot weaker.