Just as I mentioned before, it says "Kindle titles are available for US customers on Amazon.com." I'm neither UK or US customer thought, as I'm located in Sweden, and my settings for my kindle is set to Sweden to.
The .com adress of said book https://www.amazon.com/dp/B01DSTTQX6/ just says I can't buy it there either.
What does this show you? https://www.amazon.co.uk/dp/B01DSTTQX6/