Hmm... Often if it's available as a stand-alone channel as well as through The Roku Channel the stand-alone channel requires its own subscription directly through the provider, but you say you could access it before. See if you can access it through The Roku Channel. If your subscription doesn't work in either place then we have to get to the bottom of that.
To be honest, I'd cancel the subscription through Roku and just subscribe directly with BritBox. Subscribing directly through the provider allows you to use the subscription everywhere, not just on a Roku device.
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