Roku does not control technical issues that the apps have, if it is an issue with the app itself and not the Roku OS. You would need to contact Britbox support directly. On Britbox's page, it looks like that they have tech support help with 24-hour chat, email or they can call you, although I assume that has to be pre-arranged with them.
If it is device related, you could try these steps in this order and see if it helps. 1) Uninstall the Britbox app, then 2) a system restart in the settings page (or unplugging the device for 30 to 60 seconds). Powering off with the power button will not clear the cache. Then 3) plug the device back in and 4) reinstall the app and see if that helps. CS hopefully can make some other suggestions if not.
Hopefully that works for you. It is possible there is some technical issue with Britbox itself though. Maybe because you were hacked, they have locked your account or otherwise blocked it for some reason.