My Age of Imperialism history's a little rusty, gimme a break.
France to Canada as Britain is to America.
Addendum:
Give France-Canada examples and show their Britain-America counterparts to show how they compare. All for the sake of making a convincing stand that Canada qualifies as a Civ that's very culturally distinct.
Quebec-France: difficulty understanding each other due to centuries of language drift.
Quebec was
strongly catholic until 1 to 2 generations ago, then the quiet revolution took off.
France was one of the first secular states in the world. The French revolution was quite anti-religious.
Quebec was conquored and govorned by the British, and became one of the founding provinces of the nation of Canada. It still speaks a dialect of French (Quebecois), and uses a napoleonic civil code (same as France), but it has a common law criminal code in common with the rest of Canada.
America-Britian: Both are naval-power based imperial nations. Both are prodistant, with recent acceptance of Catholics as an acceptable religion in recent (post-WW2) history.
For a short period the two nations where rivals (between independence and WWI). Local dialects exist in both.
Overall, if you created the "Anglo-Saxon" civilization, it would cover both UK and America quite acceptably.
...
The reason I see that America deserves a civilization is:
1> Lots of customers.
2> "World powers" get a civilization for each era.
However, given a reasonable time distance, I'd expect the British/America era to be viewed like much the Eastern/Western roman empire: a schism in the imperial period, rather than a brand new civilization.