It depends on whats on the memory stick.
Why not charge him with possession of indecent images of children as a minimum?
It's clearly no smoking gun (I hate that idiom) in respect of him clearly being identifiable perpetrating some heinous act, so what's left to charge him with?
If this is just a build-a-case effort, then they may have shot themselves in the face if they can't make all the threads come together.
We assume they have:
- a memory stick found under a dead dog
- a phone ping (let's put the azimuth / triangulation aside for a moment)
- logged phone calls in 'the vicinty' (see above)
- a suspect who was in the general locale, indeed he lived there
- a suspect with matching deviant proclivities
- a witness statement from an erstwhile confederate / associate
What they don't have (assumed):
- a body
- an eye witness
- any DNA harvested
- any other forensics
- a confession
- corroboration of the phone calls
- not enough incriminating evidence on the memory stick (but surely enough to charge him with something?)
To make such an extraordinary appeal confirms the assumption that their version of the CPS has refused them leave to charge, so the evidence isn't enough.