I have certainly been referred to as a "penguin" by our brown clad brethren, (even though I was never in the WRAF!) and the term was often used by rock-apes to refer to non-Regiment RAF personnel, but I never knew the origins.
Warmtoast's cutting proves that it was there from the outset, but doesn't explain - "why penguins?"
Ricardian - I mentioned that system in post #29. Only ever made "flight leader" myself, although the only time I led a flight was when I was a flying officer (at which time I didn't fly!).