The United States....ranked first amongst the World's economies holds a 24.8% share contrasted by the UK's sixth place share of 3.17% share (France is seventh with 3.13%).
If we were to apply a constant percentage of GDP as a rule of thumb then it follows we would spend much more in real Dollars/Pounds Sterling on Defense.
In recent years NATO Member Nations have been reminded of their obligations to maintain each member's spending levels for Defense.....and we have seen NATO enjoying over 150 Billion Dollars increased spending.
It seems relying upon a "Whip Round" method for funding was not working.....and a polite reminder of the need for a budgeted method is required had to be undertaken to obtain compliance with Treaty Obligations by all.