There's (at least) two ways to subscribe to BritBox. One is through "The Roku Channel" and one is directly through BritBox using the stand-alone channel/app. The subscriptions are mutually exclusive and what's available on one may not be available on the other. I wouldn't be surprised if it's also possible to subscribe to the stand-alone channel/app through Roku adding one more way to muddy the waters.
In general it's always better to bypass Roku and subscribe directly with the provider whenever possible.
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