>>>Maybe it's just not reported by the victims as much
That could possibly explain a drop in figures provided by police forces, TTG, but not in the figures recorded by the CSEW. That's conducted by interviewers going into thousands of people's homes (having made telephone appointments first) and then going through a very detailed questionnaire with respondents aged 16 or over, with the data being recorded directly onto their laptops. That questionnaire takes at least 30 minutes to complete and is very carefully designed to ensure that it produces genuinely accurate figures.
Further, to ensure that crimes against young people are included in the figures, interviews are also carried out (with parental permission, obviously) with children aged between 10 and 15. Even though shorter interviews are used, they still take between 15 and 20 minutes each.
Statistic was a major specialism for my mathematics degree and I often want to throw my hands up in horror when I see the way that some questionnaires are worded (and the way that data is then extracted from them). But I've completed one of those CSEW surveys myself and I was extremely impressed with the both the way that it was worded and the way with which it was carried out.