CF - The criteria does seem odd at times. Equally our own moral compass is different which may explain that. I can sit with my son, 16, and watch a film such as Ted or Deadpool and tell myself that he is old enough to cope with it and not be disturbed / come out swearing like a trooper. Any sign of nudity however and we would be out of there. Double standards? Maybe.
At least by going to the BBFC website it explains what scenes have brought about the rating so that you can make a judgement scene by scene. For example, did you know that Paddington was given a PG rating rather than a U as at one point he hid in a fridge and that is dangerous?