Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
Roku TVs have two model numbers - one is the manufacturer model number (that you provided) and one is the Roku model number that's usually 4 characters followed by an 'X' or one or two other letters. It's shown under Settings->System->About. I couldn't find a list of supported Roku models for NowTV, so it may not matter.
What is the software version shown under Settings->System->About? Make sure you check for updates by going to Settings->System->System update->Check now.
The next question is did you create your Roku account while you were physically located in the UK? Is you Internet connection using anything that might mask your true location like a VPN? When you log in at https://my.roku.com does it show United Kingdom for the country selector all the way down in the bottom right corner?
When I searched the Now TV web site, I finally found a statement about supported devices. For Roku, they only list the Streaming Stick. I don't know what specific model of Stick is available in the UK, but it looks like Roku TVs might not be supported. You can find the info on this page down in the FAQs at the bottom.
Roku Community Streaming Expert
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Looking at nowTV forums, seems like NowTV should be supported by on RokuTV as can be installed on the Roku Express, notwithstanding the info provided by @atc98092 . (looks like can also be installed on some older Roku set-top devices as well).
In April, looks like ran into an issue and had to revert back to a previous update.
Lot of confusing info on the forum, but users report that NowTV supposedly works on their Hisense RokuTV, while others say Now Support tells them it is not supported and it has stopped working on theirs. (as of 10/2020) Could be that NowTV has decided just to develop availability for the stick and the set-top devices as reflected by @atc98092 link of supported devices. May have to check out the forum more thoroughly or contact NowTV directly. (although other complaint is having to speak to an unhelpful chat bot).