Thank you for the inquiry!
Does the issue only occur on BritBox or across other channels installed on your Roku device?
Have you tried removing the affected channel BritBox and then re-install again? If not, we suggest starting there. In some cases, removing the affected channel and then re-installing it may help. Restart your device before re-adding the channel back in to ensure the process is successful. We recommend doing this through the Settings menu by navigating to Settings > System > Power (If there is no Power submenu, proceed to the next step.) > System restart.
For detailed instructions, refer to this Support link for more information: How to resolve a channel playback issue | Official Roku Support.
Let us know if there's any difference after.