Are you entering the correct password?
If you signed up for Britbox directly on their website and pay directly to them, you should be entering your Britbox password.
If you signed up for Britbox through Roku and pay through Roku, you should be entering your Roku password.
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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Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X