It's a whole mass of things. For example Schengen area, the UK currently isn't part of this. Should Scotland gain independence would it then wish to join the open EU borders? If so, how does England react to this? It would effectively cut communities in half if it was to close the Scottish border. In terms of the nuclear weapons, I think Scotland could be convinced to keep the base on the grounds of it will need help in controlling it's fishing zones. Should Scotland seek to control it's own seas I think Iceland's fishing fleet might make an opportunistic dash for it's waters. The same can be said of it's airspace, I believe as part of NATO it would have a responsibility to secure it's airspace (against soft-touch Russian patrols), but it wouldn't be capable of it. So it would either have to allow other NATO members or allow the status quo to continue. And I don't think too many NATO members would be willing to step on the toes of the existing forces, unless we saw it as a cost saving measure. I don't think Wales will push for independence in my life time, unless we become the home of Fusion energy or something and have something England needs. Or perhaps fracking in the short term (2-3 decades).