All current economic statistics about Scotland are baed on origin. Anything made in Scotland is counted as part of Scottish economy, even if it travels to England to be exported—that is counted as a Scottish export even though it left via an English port.
The difference after independence would be that the export would occur at Carlisle rather than Felixstowe.
Most of the "Thatcherism effect" of the 19080s can be directly attributed to North Sea oil. If you remove that money from the economy Thatcherism was a contraction until the banks boomed (though that was mostly post-Thatcher).
How would the borders go in case of separation? Seems Scotland has only one refinery, and it's half owned by a Chinese company.