Your Roku account can be used as a payment method for BritBox through two options: by subscribing directly to BritBox or by adding a premium subscription to BritBox for The Roku Channel.
These are two separate subscription options which can't be used interchangeably, regardless of your payment method.
Example: you subscribe to BritBox directly, and pay using your Roku account. You watch at BritBox.com or in any of our apps, such as our app for Roku. You won't be able to watch BritBox for The Roku Channel, but you will be able to watch in our app for Roku.