Interesting post Neil on how you see the Ex Pats health treatment being affected by a leave vote but reading the links thatI have posted below would suggest that a cheap flight 'home' on a budget airline for treatment, could still be an option for many.
Unless I have misunderstood the content of these links, I would suggest the entitlement of EU members to treatment on the NHS could spell the end of free treatment at the point of delivery for everybody, including UK residents, should millions from Turkey and other potential members be allowed to join the EU and take advantage of it.
Imagine the cost to an already underfunded NHS.
https://www.angloinfo.com/uk/how-to/page/uk-healthcare-health-system-eligibility
http://www.nhs.uk/NHSEngland/AboutNHSservices/uk-visitors/moving-to-england/Pages/moving-from-outside-the-eea.aspx