Barn,
Public sector offices have been steadily encouraged for years to be more business-like, whether through relatively soft arrangements like JBAs, Smart Contracts etc, or direct billing and accounting for services. Assuming a geographical divide of the North Sea for the new EEZ of iScotland resulting in the new iScotland owning approx. 90% of the oils fields, then if Scotland were unable initially to provide the defence required of the rigs which provide 10%+ of income, it would seem perfectly feasible for rUK to offer the services at cost.
rUK maintaining an effective military force and partly subsidised by the new iScotland doesn't seem so bad a deal.