All cases will need to go through CPS first to 'test' the evidence to determine any realistic prospect of a prosecution. There also has to be an element of things being proportional and in the public's best interest.
For example - two children under the age of consent, engage in sexual activity, both confirm what had happened and that they both agreed to do it. However, they can't consent due to their age - Is this in the public interest to prosecute through the court system when you have a more appropriate out-of-court disposal you can use?
Another example - male patient in a mental health hospital, has full capacity for his decision making, puts his arm around the waist of a staff member. Staff member feels as though he has done this for sexual gain. Is it in the public interest to prosecute the patient?
School pupil gets sent an explicit picture. She then send this on to a friend with a text of "I CAN'T BELIEVE WHAT I HAVE JUST BEEN SENT!". Do we prosecute her for distributing explicit images, malicious communications. Do we prosecute her friend for possession of explicit material. If you want the police to do their job properly, both girls will have to be arrested and their phones downloaded. Is this proportionate, or is there a better way to deal with this without the need to go through the courts?