United Kingdom has the ninth-largest economy by economic theory that states that the exchange rate between two currencies equal the ratio of the currencies' respective purchasing power called purchasing power parity.
Purchasing power parity ( PPP ) is an economic theory that states that the exchange rate between two currencies equal the ratio of the currencies ' respective purchasing power . The concept of purchasing power parity allows one to estimate what the exchange rate between two currencies would have to be in order for the exchange to be at par with the purchasing power of the two countries ' currencies . United Kingdom . The United Kingdom is a developed country and has the world 's fifth-largest economy by nominal GDP and ninth-largest economy by purchasing power parity .