The "Can-UK" theory seems very plausible to me. I hadn't heard it before. The only thing that would dissuade me from believing this would be evidence that the term predated WWI.

If true, of course, it would have referred to all Canadian troops, both of French and English heritage/language, since all were Commonwealth citizens at the time.

I found it interesting so see a Canadian French-Canadian opine that the terms applied specifically to Anglophone Canadians; in the US, I've known the term to apply both to Canadians in general and specifically to French-Canadians (or perhaps more exactly, to American citizens of French-Canadian descent living in New England, including the author Jack Kerouac).

Incidentally, I don't put any credence in the argument that the name of the NHL team in Vancouver has any relevance to this discussion; those Canucks are an expansion team, which adopted that name much too recently for it to mean much except that the team is representing themselves as "Canadians" (not "Canadiens," like that other, much older, team in Montreal)...