In: Economics
For developed nations, the illicit activities account for around 10% of the GDP. Thus, if such activities are counted in GDP, this will increase the GDP numbers for that country, surely. The problem with the underground activities is that underground economic transactions go unreported, which distort the accuracy of a nation's gross domestic product, which consequently may adversely affect a government's monetary policies.
Since in this globalised world, all countries are interrelated, thus, increase in GDP of UK, and distorted monetary policy, alongwith instability in the market will impact other nations as well. The trade will be impacted with UK.
The inclusion of such activities will bring some uncertainties in GDP calculation. So government should include such activities only after deep research. The tax structure should be improved in order to encourage people to report their income accurately.