Gross Value Added is a UN stat, if memory serves. It's like value added for the purposes of VAT, ie selling amount less cost of production/inputs, but with an added twist for national accounting - it also deducts national consumption of the good or service produced.

Since much of UK production of gas and oil is also used as intermediate production inputs for other goods and services, this would discount the portion of oil and gas production on those portions not consumed (ie not ending up in another state as finished goods or services) or exported. And, since much of the transformation of oil and gas as intermediate good occurs in England, then the understating is simply compounded.

Account for it the way countries are accounted for rather than regions within the UK (and, perhaps for further measure, put Scotland on the Euro) and I strongly suspect Scotland's GDP would be closer to Norways' than the GVA statistic London puts out.

The Hun is always either at your throat or at your feet. Winston Churchill