Originally Posted by
Miles Magister
Tax agreements are between national countries and are not part of the EU so are completely unaffected by Brexit
That's is of course true, it's the the provision of reciprocal reciprocal healthcare arrangements that either has or potentially is going to become more complicated.
Edit to add, for benefit of OP:
To my knowledge current contact number for UK NHS Business Service Authority with regard to querying entitlement to reciprocal healthcare, UK/NL and the possibility of obtaining an "S1/Portable Healthcare Document" is:
+44 (0)191 218 1999
Hope that helps, Good Luck.