I've never seen Vitamin C listed as E300 on a label. So while you are correct, the heuristic (avoid E-numbers) works in practice.

OK but then why would a manufacturer ever use a E number - considering the stigma attached? Only when the other name "sounds worse"?

Is there a legal threshold where you have to use the E number?

Indeed, they’ll use E numbers to shorten the chemistry catalogue part of the list. Note the law requires sorting ingredients by weight, so these additives end up clumped together. You’ll end up with half a line instead of half a page.

Limited space on the label maybe :D Some of the real names of the E's are very long.

Yeah, anything good wont be listed as an E number, you use the name people recognize.