When watching videos people seem to equip very little, usually Tawny owl and food or wine.
I would have expected every one to have thunderbolt and superior swallow equipped.
Presumably there is some advantage in not using potions.

I have just viewed a video "Beyond hill and dale"
With only two bottles of wine equipped Geralt's health bar becomes totally empty fighting pixies (he should then be dead)
then suddenly his health if full again even without consuming any food/wine.

Any feasible explanations for this?

Restorative Quen? For his health to return from near nothing to full, the player *must* have used some method, even if it's cheating. It doesn't suddenly return to full on its own.

There is a very simple explanation, excluding possible glitches in the game or in the video

It's called Cheat Engine, a script that make Geralt immortal, probably not even coded particularly well if it provoked a glitch on the health bar

Alternatively, a mod that put Geralt health minimum value to 1, effectively making him impossible to kill, with a ridiculously high regeneration value for health, so high it's seems instant

Invest 5 points in "Undying" (combat branch).

If the adrenaline bar went down as the health bar went up, then the player most likely has the Undying ability equipped(If that ability has 5 points invested to it, health is replenished at the cost of adrenaline points). Since its towards the end the main quest of Blood and Wine, the player may have the special Blood and Wine mutation called Second Life.

Or, they may be using cheats or mods.