Well, yes, that is the scholarly consensus. For 3 reasons: when they are from tightly dated sites, they are Brit era sites, when they have marks, they are only Brit marks, and 3rd it is a know Brit type.
Refs:
Museum of the Fur Trade Quarterly, v. 23, No. 2, Summer 1987. "The Cartouche Knife" By Charles Hanson. Pp. 1-4.
Fur Trade Cutlery Sketchbook by James Hanson. He dates them 1780–1825. Definitely not French era.
There are plenty of these knives from late sites, as far away as Rocky Mountain House, that prove they are Brit. The only mention of French I have seen is in Stone, where he says the scales "might represent French clasp knives." Well, you know how rare these are as clasp knives, and the 2 I have seen had Brit blades. The cartouche knife in general (all handle types) is not associated with French posts, or like French-era-only Native sites.
Quimby doesn't have much on knives. Too bad.
If you have Brit marks I can ID them with a maker name and even date some of them. If I can't I know 2 people who can. Except that one with the crown and a plus and cross keys. Yeah you know that one. Its Brit but nobody has tracked that one down yet. It shows up before 1781 and as late as 1804-5.

Maybe a London mark they don't have their records anymore. Sheffield does.
