The problem with that measurement is that top GDPs fall off quickly. While the UK is the sixth, it's only around 3-4% of the global GDP, and while I can't find values from the same year at the moment, likely has a lower GDP than California. The EU is around 18% of the global GDP; the US, China, and EU together make up around 65% of global GDP, and each of them has at least triple the GDP of any other individual country.
Most of that money is bound in the financial industry, though. Unless you're in the sector, the GDP rank doesn't matter much for doing business with the UK.
The UK exists right now as a shell company City of London who are the tax havean bankers of the world. The UK knows your financial secrets, and thats how they keep themselves relevant.