Form S1 gave UK retirees in the EU and EEA access to reciprocal health care.
ONLY as long as we remained in the EU. That was the deal.
The storm clouds regarding our EU membership were gathering at the beginning of this century, if not before.
There were NEVER any guarantees.
EU membership or not, policy changes regarding the NHS (the like of which doesn't exist elsewhere - or in very few countries) may well have been implemented - to affect those living abroad. Moving away involves a loss of things that are familiar, which includes engaging with foreign health care systems. It is a gamble and the EU has been wobbling, for various reasons, over the years.
It's powers have also been questioned throughout Europe. Many feel that it will collapsed in the not too distant future, Brexit or no Brexit. Read the arguments.
The NHS is not a bottomless pit of finance, equipment, specialists etc. We all know it is long overdue a thorough overhaul.
Those people living in Europe will have to consider their options. Every time we travel abroad healthcare insurance costs us a fortune because of our pre-existing conditions. We have to pay it, or stay put, or gamble on travelling without it, which we wouldn't do. It is extremely expensive. It's the price we pay for aging and being more in need of health care unfortunately. I imagine that applies throughout the world, unless anyone can tell me differently.