Devolved government functions explain Scotland's higher per head spending on Public & Common Services
Because reserved government functions are almost exclusively allocated on a per head basis, Scotland's higher spending per head is largely explained by the costs of running the Scottish government.
If Scotland were to separate from the UK, what it would cost to operate dedicated, stand-alone government departments to replace those whose costs are currently allocated to Scotland on an 8% population share basis (e.g. HMRC, DWP, Home Office etc.) is an open question.
Costs allocated to Scotland for the House of Commons and House of Lords are often cited in debate. These are allocated on a population share basis (so do not contribute to the observed difference in spending per head) and combined represent only ~£15 per head within the GERS figures.