Raw economy size can be misleading in two ways. The value of a dollar is much less or much more depending on where you're at. So an economy of 10 shekels might mean an economy of 100 widgets, or it might mean an economy of 1 widget. Purchasing power parity (PPP) attempts to account for that. The second is that economies are largely a product of population. An economy of a million making a million shekels is quite a bit different than an economy of 10 making a million shekels, so you also want to look at per capita values. Even both of these adjustments combined [1] can be extremely misleading (see: Ireland and many other places...), but they provide at least a less unreasonable basis for comparison than nominal dollars. And the UK is currently 30th there.

[1] - https://en.wikipedia.org/wiki/List_of_countries_by_GDP_(PPP)...