I would suggest canceling through Prime, then subscribing directly with Britbox on their website. Then you can view it through the Britbox app on any platform for which it provides an app, including Roku, Android or iOS device, or computer on the web.
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