How do I add BritBox (or any other streaming service) to Apple TV?

Depending on what is available in your region, you may

- install the BritBox app and sign in there with a BritBox account, for watching in the content provider app on any of your devices,

- or subscribe in the BritBox app through Apple, for watching in the content provider app on any of your Apple devices,

- or subscribe in the Apple TV app to the Channel BritBox, for watching in the Apple TV app on any of your devices.

These types of subscriptions are not exchangeable without canceling and re-subscribing, so select how you want to watch carefully: in the content provider app, or in the Apple TV app.

Find apps in the Apple TV App Store - Apple Support

BritBox app - App Store preview - Apple (US)

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

I already have a subscription, but not thru Apple.

If you subscribe with BritBox directly, then get/install the BritBox app from the App Store, and then sign in to the app.

Find apps in the Apple TV App Store - Apple Support

If you subscribe with a third party channel reseller (e.g. Amazon), then you may need to do as they say (e.g. use the in-app Channel of the Prime app).

With “outside” subscriptions, you can't use the in-app Channels of the Apple TV app.

Neither do I see the BritBox app listed as an app that can ‘connect’ to the Apple TV app. Availability of supported apps in the Apple TV app - Apple Support