It sounds like your Roku account isn't associated with the UK (probably US). The only way to fix it is to create a new account ensuring you're not doing anything that might mask your true location like using a VPN. Close the current account and create a new one. You might want to use a different email address, but make sure before you create the new account you clear your browser cookies or use a private/incognito browser window. You can also try a different browser or different device. I wish I knew a surefire way to ensure an account isn't inadvertently being created in the wrong region. It's far too easy to do.
Once you have the new account, factory reset the TV and start over. Hopefully the UK channels/apps will show up, but be aware that if you imported the Roku TV/device some UK providers won't recognize it as an acceptable device.
How to close your Roku® account | Official Roku Support
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