The first thing to check is whether all of your Roku devices are linked to the same Roku account. Check the email address that's displayed on each one under Settings->System->About. Subscriptions through Roku are linked to the Roku account, so if the bedroom TV is linked to a different account you won't have access to the subscription. After that, there's two possible ways to access BritBox - through their standalone app and through "The Roku Channel". They aren't equivalent and if you search for "britbox" in the "Streaming Store" you'll see both of them - one of them has "on RokuChannel" on the icon and one doesn't. You may be using one on the working TVs and the other on the bedroom TV.
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