It would be fascinating to apply BBFC criteria to plays and books
I think it would be a crying shame to do so. Our minds handle books - and to a lesser extent, plays - in a more abstract way than they do films.
Part of the reason plays have managed to escape classification is because it's still so expensive to go and is consequently mainly a reserve of the middle class. Add to that the fact that parents take their children, children rarely go alone.
Whereas the cinema is relatively cheaper and kids go with their mates so there's no prepping or discussion about what they're about to see or what they have seen. Slap a 12A or 15 on it and the discussion becomes obsolete for some parents.
Sometimes I think I'm against current film classification rules. Obviously I think it's fine to have restrictions on things for young children. U, PG, 12A are sensible rules, if parents use the 12A guideline sensibly. But once a kid gets to about 14-15, I think there are many vagaries and it gets more complicated. the difference between what a person can handle at 15 vs what they can handle at 18 are nuanced.
I saw a brilliant film last year called 'Diary of a Teenage Girl' which was given an 18 rating for the graphic sex scenes. But it was a film about a 15-year-old, and I think a lot of teenagers would have benefited from seeing it and the questions is posed about relationships, self-awareness etc.
I think our threshold for tolerating violence in film is lower than our threshold for sex, which is wrong to me. Violence is always violent; sex, presented in various ways with various consequences shown is more a real part of life and yet it's pushed to the top shelf.