Britbox is tricky on however it is you signed up. I signed up through Apple TV and can only watch on Apple TV and not the Britbox App. I can watch on my iPad or my Apple tv4k and it carries on where I left off on either device, although I find the menu hard to navigate. I’m in Canada as well.
I can’t find where I read the parts telling me this but it’s on their Support page someplace.
From their support page: https://www.britbox.com/ca/help
If you would prefer to access on other devices you can also sign up for a BritBox subscription separately through Amazon Channels or Apple TV Channels. Amazon Channels supports all major device platforms including FireTV, SmartTVs, Blu-ray players, Game consoles, and Fire tablet. Apple TV Channels supports all major Apple device platforms. See all supported devices from Amazon HERE and from Apple HERE.