That is fully supported, and not dependent on your Apple TV+ offer through T-Mobile. These are largely independent of each other, although it can happen that certain subscriptions are on offer for slightly less when in combination with Apple TV+ or other services (not sure if that ever applied to BritBox).
BritBox availability is dependent on country or country-like region, and may not be available outside the subscriber region (e.g. on international travels).
There are several possible ways to subscribe to BritBox, each with their pros and cons. Find out what suits you best. These options would not be exchangeable without canceling and re-subscribing in one of the other ways.
- You can subscribe to BritBox in the Apple TV app as in-app channel, requiring the use of the Apple TV app to watch this content. Subscription payment is through Apple. Subscribe to Apple TV channels in the Apple TV app - Apple Support
- You can subscribe to BritBox in the BritBox app on an Apple device, requiring the use of the BritBox app on any of your Apple devices to watch this content. Payment is through Apple.
- You can subscribe to BritBox with BritBox directly, requiring the use of the BritBox app (on any supported platform) to watch this content. Payment is through BritBox.