Ok, so there's two other possibilities I can think of (three if you're using some kind of VPN to make it appear you're in the UK). Given the model number of your Roku it appears you may have set it up in one location (Mexico) and then moved to the UK. The other is that your model isn't supported. Going from memory, I vaguely recall some issues with people using the various UK channels and those channels only being available on certain models of Roku devices. Why it worked and then didn't I couldn't say.
Amazon not working points to some issue with your true location or a mixup in where your Roku account was created and where you're currently located.
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