That's been tried before! It never ends well!!!!
Seriously, I think it would take years for Scottish separatists to be in any position to be a serious threat to England and by that time I'd imagine the entire BAOR would be back in the UK and working on restoring HMG's control. I don't see the Scots being able to take on the BAOR and win, so we'd lose. Again.
With regard to Wales though, I certainly agree that there could be some WNVA troops in England for the reasons that you mentioned...in my alternative piece I placed a detachment of Welsh troops in Oswestry in Shropshire.