Britbox subscription through Apple TV on Roku

• The subscription for the Apple TV app Channel BritBox has to be started in the Apple TV app, and then can be viewed in the Apple TV app on any of your devices.
• Subscriptions started outside the Apple TV app will have to be viewed in the separate BritBox app.
• In-app subscriptions on Apple devices (for the separate content provider app) are administrated with an Apple ecosystem specific Subscriber ID (under the hood), so that a content provider doesn’t need to know your Apple ID account and password. Because of that, such subscriptions are generally not recognized on third party devices.
• Subscriptions with the content provider directly require an account with that content provider, with username and password, and then you’ll need to sign-in to the separate content provider app.

You can cancel and re-subscribe to get the subscription type that better suits your viewing devices and habits; either the Apple TV app Channel subscription, or with the content provider directly.

Subscribe to Apple TV channels in the Apple TV app - Apple Support

Apple TV Channels FAQ: Services, pricing, availability, and everything else you need to know - Macworld