Getting a bit offtopic to you guys' discussion on Avalon I think the END parameter accounts armor. They note Saber Alter's armor got heavier for example and this reduced her AGI but increased her END.
Likely the parameter wouldn't cover something like her helmetless face. And Artoria's armor did sustain a blow from Berserker in the Fate route before it broke to pieces because she didn't have enough Mana to repair it.
Likewise in Zero she scored a direct blow to Lancelot's head and the sword bounced off. If it was straight to the face it should be lethal.

Of course that's kind of case by case basis including if it factors an NP or not. Karna and Siegfried's END parameters don't seem to be including their respective Armor NPs in Apocrypha (specially because Karna's END is C, while in Siegfried's case they note the resulting E-ranked attacks that damage him are essentially harmless scratches). And we know Heracles is really fucking sturdy even when an attack bypasses God Hand and he is half-naked.