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.
According to this thread, people have contacted Britbox support and they've said they are working on something in the "near future". (But they said that over a year ago so that may not match your definition of "near". )
The channel apps are created, provided and maintained by the channels themselves. Roku just provides the platform on which they are run.
There are two steps to a channel becoming available on Roku in any given geographical area.
The channel owners must authorize the distribution of their content in that region. In many cases, the channel must license the ability to distribute the programming into that area from the actual owners of the content, and some other licencees may already have exclusive rights in that region.
The channel owners must create a version of their channel app for the Roku that is configured to work in that region and provide it to Roku for distribution. Each major streaming service (Roku, Amazon Fire, Google Chromecast, Apple TV, etc.) will require its own specialized version of the channel app.
If a particular channel is available in your region via other devices but not on Roku, then it sounds like the channel has not yet released a Roku version that is enabled in your region. I would suggest you contact the channel about this directly, but don't be surprised if the front line people at the channel doe not understand this.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T