It should basically be impossible, though. Such an early war with the UK should mean your entire coast is blockaded, pops can't get their needs filled and get pissed and rebel, 100k troops invade from canada, and then 50k troops land in your capital after the royal navy kills your navy. UK was an absolute monster in vicky 1, as it should be. Is the AI simply not garrisoning Canada at all? Or does the UK just have no troops or something? Really makes no sense.Bear in mind that this was one man's game in - what - ten thousand? In my game I barely managed to white peace with Mexico, let alone even consider any sort of gallivanting about Canada. But then again I'm a Victoria n00b.
Maybe all their forces are on the border with China? I mean China has hundreds of divisions, so they are probably massing a bunch on the border, which then makes the UK AI mass all of its divisions there as well.