If you have a specific case you can just (well, I can't easily but a lawyer will know how without having to go re-read documentation they didn't look at for years) get the court records for that case and see what happened to it. The CPS claims they can't answer a FOIA "what happened to these cases?" type query because once the CPS owns the case it's just another case, but if you had a particular case in mind that's not a FOIA situation you just look at the court records.

From the sound of the things you're thinking about what I'd expect is that the defendant tells the CPS, the CPS seize this case and then they have it dismissed. If you weren't looking for evidence it will be pretty easy in almost any case to decide you do not have sufficient evidence for reasonable prospect of conviction, but in such cases you can probably also make out a position that there is no public interest in such a prosecution - for example because it would be embarrassing.