The Office of National Statistics calculate all the economic indicators to arrive at the figure for GDP, which is important as a guide to how well the economy is doing. All Governments use these figures including (when we get into Power) the Loony Party.
However they are now including estimates from income of Drug dealing and Prostitution to help gain an accurate figure.
We in the Loony party would also include estimates of incomes from:

• Rape and Pillage
• Parking Ticket receipts
• Nector points and Tesco loyalty card receipts

You know it makes sense