In general if you subscribe to something through Roku you can only use that subscription on a Roku device. If you want to use a subscription on multiple platforms it's better to subscribe directly with the provider. Unless Britbox provides a way to use your Roku credentials on your computer (which you'd have to ask them about) it's probably best to cancel your Roku subscription (which would still be valid until the end of its current term) and subscribe directly with Britbox.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.