Thanks, a good thought.
Something else that occurs to me is that all the people identified so far are high-echelon freemasons. Another one I've identified today in the photo is William Howard Taft, one-time US president, who was in the UK in 1922 to work on a justice system - he's the chap in the boater, and he's a mason.
Also Googling around, I find that the Duke of Connaught popped up fairly regularly at horse shows, including one in France in 1917.
Paddy