Modern Hairstyles for a 1930's Setting

Anna Maxwell Martin was very good in this but I felt her hairstyle was highly improbable for the 1930's. Why are some actors so reluctant to cut their hair for accuracy when many others will take their parts seriously enough to lose or gain considerable weight? Several others - particularly the schoolgirls (at least they had an excuse they were probably paid next to nothing so why bother) were also guilty of this but the worst of all may have been Mr. Holly, Lydia's father who lives in squalor yet had a nice modern haircut (and even clean, attractive clothes. Why do actors accept period pieces if they are not willing to allow their hair to be cut in styles appropriate to the era? Of course, the actors are not really to blame, it's the producers who don't insist on it.