How did you subscibe to Britbox?
If you subscribed directly on the Britbox website, install the Britbox channel app to your Roku, and access and log into it there using your Britbix credentials.
If you subscribed thru Roku, access it through The Roku Channel.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X