I have mentioned before the tendency (a.k.a. rule #1) for those advocating secession from advanced democracies to promise that the enterprise will be relatively cost-free. Why? Because they need votes to win a referendum, and if they can just promise unicorns and candy, or just promise better identity protection (French, Scottish, whatever), without the new country having to start from scratch, they might get more votes.
But the reality is this: if you become an independent country, you are no longer part of the old one. Yeah, that is pretty much true by definition, but the idea that you can keep the good stuff and dump the bad stuff is just not reality-based. The rump country will treat the citizens of the new country like ... foreigners. As much as we get heaps of integration (NAFTA, EU), those invisible lines between countries mean something.
So, the latest "surprise" is that if Scotland leaves the UK, it will no longer be covered/part of MI-5, MI-6, the Five Eyes (the intel sharing community much in the news lately---US, UK, Canada, Australia, New Zealand), etc. I have already discussed the need for the Scottish separatists to start changing their tunes on NATO and nukes.
Of course, the UK folks have an incentive to make independence appear to be more costly and to diminish the apparent benefits of leaving while making sticking around more attractive. The reality is that most of the clubs that the Scots (or the Quebeckers) would suddendly find themselves out of would eventually let them back in. Ok, most of them but not all, as the parliaments and Prime Ministers would no longer have any incentive to play to the audiences that are now outside their country. Scotland would get back into NATO, but I am not sure how many UK tax dollars would be spent on helping Scotland be secure via various UK intel services. Maybe some but certainly less than the status quo ante.
Perhaps I am status quo-biased, but in these kinds of arguments about leaving while keeping the goodies, I tend to find the host state more credible than the pie crust promising secessionists. Becoming independent has costs. The problem that separatists in advanced democracies generally have is this: the democracy is not that oppressive (or is at all oppressive), so the benefits of becoming independent are not nearly as bountiful. The costs, then, become important. In other places, where one is seceding from a brutal, repressive, unrepresentative government, then no one argues about which organizations or services will remain intact. The idea is to get out regardless of the residual benefits because the costs of remaining are quite significant.
The funny thing is that the separatists get caught in a dilemma--they have to argue that secession will be meaningful to attract the more passionate nationalists but then they also have to ague that it will not matter that much to get the softer nationalists who care about identity but not so much that they want to sacrifice much.