Assuming you're in the US, you should be able to add it from this link:
https://channelstore.roku.com/details/ae9a77e6a02e540039dccfcd6fc53706/britbox
However, if you're paying for a subscription through Amazon don't be surprised if that subscription only works on Amazon devices. Can you sign in on their website?
https://www.britbox.com/us/
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.