Hey neighbors. Due to regional requirements, smoke detectors are currently not available for the Ring Alarm system in the UK or EU. Different regions utilize different Z-Wave frequencies and have different requirements that need to be met for those Z-Wave devices.
With that said, you can add your feedback for the availability of a Z-Wave smoke detector in the UK and EU on our Feature Request board. I found the request below asking for this, which you can comment on and vote on. Thank you for your feedback! 
https://community.ring.com/t/smoke-detector-option-in-uk-app/86581