Thanks for the note. I would suggest removing the channel from the Roku home screen once more, restarting your device, then installing the channel once more. https://channelstore.roku.com/details/143088/britbox
When you first launch the BritBox channel, there should be a settings menu in the far upper righthand corner (looks like a user icon) which has an option to 'Sign-in' within. Click on this option. You may see a pop up asking you to create an account, but you should be able to click 'Cancel' and return to the option you just selected and manually type in your email/password.
If you don't remember your password, try clicking the 'Forgot password' link and put in your Roku account email address. You should receive an email thereafter which will assist you in resetting your password. Then, you should be able to login from there.
Let us know if you need any further help!