I recall was a very good BBC documentary series years back following officers who had to do exactly that. the operation that caught Pete Townsend among others.I known the offences relating to this are categorised for a reason but I struggle to get my head round the idea there's a worst form as such.
Also, investigating and allocating these images must be hell on the mental health of those police officers doing so.
they talked a bit about coping strategies and the support needed to get through.
I don't know if it's ever been repeated unfortunately.