US: 1750-2155 deaths, 330,000,000 people
UK: 448/63 million
Canada: 157/33 million
The back of my envelope says the UK took the highest ratio of deaths to population. US and Canada were next in line. For comparison Germany, with 80 million people, took 57 killed. Far less than the UK with 63 million people. Either the Germans were highly skilled at not getting killed, or they were not involved in heavy combat like the other three countries cited.
Remember for most of these countries ISAF stood for I Saw Americans Fight.
That was the only time NATO forces were actually used in combat operation on a request of one of the members, so keep your trap shut.
Did some calculations.
Deaths per million population
USA 2315/313 = 7.4
UK 448/63.2 = 7.1
Canada 158/34.9 = 4.5
I understand that you want to ignore small countries because they are small?
Denmark 43/5.6 = 7.7
Estonia 9/1.3 = 6.9