Background: In a world I am building, France becomes a much larger power in the New world, actually maintaining control of French Louisiana and even gaining control of Canada during an alternate version of the War of 1812. Canada and French Louisiana eventually merge to form the French Republic of Colonial Canada (or the FRCC), when the FRCC attempts to become an independent nation of its own in the late 1870's, France refuses and the Canadian War of Independence occurs.
The people of The United Dominion of Canada realize that while they are strong, the French Homeland is stronger, they need an ally. England is the best bet, but they are less than willing unless Canada decides to join the Commonwealth. Long story short, Canada take the deal and wins the war.
The problem is that my lack of knowledge on the commonwealth and on England as a whole, means that I am making too many assumptions; That England would make such a deal, that such a deal is possible and that a nation would even consider this deal.
While I'm having a great time imagining this Commonwealth Franco-Canada, I do not think it is even legally plausible to do so. All I need (or want) is a plausible explanation if the above is doable and if so, how I can make it so.