I doubt the KC would care - we bought Chloe from one of the top KC breeders - turns out the dogs were a result of two family pets getting out together, she came home absolutley swimming in fleas...they obviously don't care that much! xxxx
If it is an accredited breeder the KC will look into any complaints.
I dont know what the issue is, but if it is about the health or welfare of the dog you could report the owners to your local council or dog warden - they are much better at responding to these things than the RSPCA.
Ditto above, if its an accredited breeder the KC should take action if you believe what they preach, otherwise I wouldn't hold your breath. If the breed has a breed council or similar it might be worth contacting them asking for advice. If its welfare as Una says the dog warden is often a good eprson to speak to.
SKK have a disciplinary board, I've not had any need to think about how to contact them so I don't know precisely how to do it, I suppose I would simply phone or write to someone involved with information or legal things on SKK and ask for advice about how to proceed. I can only suggest that you do the same but to your Kennel Club?
Though besides reporting it to your Kennel Club, like CaveCanem, I like Cayla's idea about informing the taxman too.
What breed is it? It is worth reporting it to the KC with a vet's statement on the health and reasons for the PTS. If the breeder is on the ABS list they may well barr them. Every person who stands up now against breeders like this will help in the long term.
Also send a copy of the report to the breed society concerned. Some of the various breed societies do care and will stand firm on health issues.