The Australian historian and journalist LA Carlyon in his book Gallipoli reports Australian troops, a higher proportion of whom at that date had grown up in a rural, outdoor life (the same was probably true of Canadians), noticed that British troops who had grown up in the then smoky and crowded industrial cities of Britain often seemed less well nourished and healthy, even stunted in intellect.
This is not to say that the Australians were without faults. Vera Brittain who wrote about her experiences as a Nurse in World War 1 in 'Testament of Youth' said that if there was any trouble Australian troops were always ready to be part of it.
British War Correspondent Philip Gibb in his book 'Now it Can be Told' said that of British Empire troops the Scots and the Australians were more likely than others not to take prisoners but to kill enemy troops who had surrendered.
Britain recruited a higher proportion of its adult male population into the armed forces during the war, including by conscription from 1916, which meant that the British army probably came closer to 'scraping the bottom of the barrel' and taking some less suitable recruits. Australian, New Zealand and most Canadian soldiers were volunteers, hence more likely to be motivated.
Of course we should be cautious about accepting every generalisation made then or now about the qualities of soldiers of different nations, which can be biased by patriotism, stereotypes or chance as to e.g. which Canadian soldiers someone met and under what circumstances, as they were all individuals and doubtless some braver, more intelligent, better trained etc. than others.