While the UK is in the EU it means that  healthcare for UK citizens within the EU is currently enabled by EU Regulations. After December 2020, (if there is a transitional arrangement from 29th of March 2019 which would protect the current arrangement for a short period), the UK would no longer be part of the reciprocal health care arrangement. That means UK citizens on holiday, working, or who have retired to an EU country would no longer be covered.
https://theorkneynews.scot/2018/11/14/what-happens-if-you-require-medical-treatment-in-reu-after-brexit/