We can’t tell anything useful about Scottish GDP because:
Why might the data be misstated? First, there simply isn’t enough data to reliably estimate Scottish GDP. We have no figures for where sales take place in the UK, for example. VAT returns are an utterly unreliable source for this: a UK company does not submit data separately on sales in Scotland from elsewhere. The same is largely true on spending. So forget Scottish GDP data: we just don’t know what it is.
It’s entirely true that it’s difficult to track sales by location inside a customs union or single market.
But then we don’t measure GDP by sales anyway. We measure by production, consumption or incomes, each of which should be, but won’t because of cheating, the same.
And we can measure place of production, place of consumption and place of the person earning the income.
Which is why ONS uses the income and production approaches to measuring GVA, GVA being the equivalent of GDP in smaller statistical units than that customs union or single market.
Shouldn’t a professor, even one of practice, in international political economy, even at Islington Technical College, know this?