Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
I already answered this months ago, but I will answer again to help others.
I had to subscribe to Britbox on my computer first, provide them with payment info etc....After that, I could connect it to my Roku. So to make it clear, I could not subscribe to Britbox through Roku. You have to go directly to Britbox, enter payment info, and then connect it to your device. I hope that helps everyone, and saves everyone time, and peace of mind.