Factual error: Jack's RAF uniform is wrong on a number of levels. He is not wearing the eagles at the top of each sleeve worn by all RAF airmen. He is not wearing the correct Air Gunner brevet (with a single wing), but in fact wears a made-up badge that is claimed to indicate he has shot down five enemy aircraft. He does not have a cap badge on his forage cap.
Factual error: Jack could not be demoted from sergeant to corporal by his CO. Substantive NCOs can only be demoted by court martial. As aircrew, his rank of sergeant would be a substantive rank, not an acting one (since it was the minimum rank for aircrew). And rank chevrons were made in one piece (two for a corporal, three for a sergeant), so if he was demoted he would have a new set of chevrons, not the top chevron removed as depicted (not only would this be impossible, but it would be a clear indicator that he'd done wrong and given he is still an NCO this would be demeaning).