If a medic inflicts physical punishment on a patient for whatever reason, is it criminal assault?

That would depend on the situation IE: if the medic was trying to deal with some drunken low life and that person kept attacking the medic, or a friend of that person attacked the medic, then no. It will come under self defence' If the person was on drugs it would be the same. No one can really answer your question until you give details. Medics and fire services are regularly attacked by ferrel sub human garbage and street trash now days. medics do not inflict punishment. That's not their thing.