Perhaps they allow us to join, as maybe they have "special" jobs for us?
"allright, you boys in commonwealth squad go in first...we'll wait here and ummm...cover you..."
-Anyone seen South Park the movie? Operation: Human Shield?
I imagine the British army needs plenty more recruits than commonwealth countries? Wouldn't they be one of the world's leading military forces? There are plenty of jobs that you have to be a british citizen to do. I don't think I could be an officer, and there are a number of soldier jobs that have more restrictions on them as well. Perhaps they save the more important jobs for you guys, and make us do mostly gruntwork? "allright Canuck, your mission is to dig that trench there, while we supervise...it's an important job, so get to it"
"Yes, Sir!!"