Unfortunately, you can't use your Amazon, Apple TV Channels or The Roku Channel subscription to access the BritBox.com, including our apps for mobile, tablet and TVs.
You'll only able to access and watch the BritBox service where you signed up.
Example: You subscribe to BritBox via Amazon Channels. You won't be able to log in using your Amazon account at BritBox.com; you'll only be able to watch through Amazon Prime Video.