https://en.wikipedia.org/wiki/Stiff_upper_lip said:
The phrase is most commonly heard as part of the idiom "keep a stiff upper lip", and has traditionally been used to describe an attribute of British people in remaining resolute and unemotional in the face of adversity.[1] A sign of weakness is trembling of the upper lip, hence the saying keep a stiff upper lip. When a person's upper lip begins to tremble, it is one of the first signs that the person is scared or shaken by experiencing deep emotion.[3]
I guess when faced with a problem, not allowing oneself to be overtaken by emotions in order to face the challenge is a good thing. But then following the event, i imagine it is actually counter productive to block oneself from feeling the emotion - meaning that when it's 'safe', it is good to 'let it out'. Whereas i think the British idea has been to resist feeling or expressing anything at all, even once the event has passed. Well, i'm British and that's the impression i have.
And considering the Development and Trauma book and what it says about where tension is kept because of trauma, how there are diaphragms around the neck, knees, feet, and so on, maybe there is something to the face and the lip area holding similar tension.