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.