Kennel club breeders can only breed kennel club breeds though.
It's much more common nowadays for people to want crosses, e.g. the Cavapoos, Golden doodles, Maltipoos etc.
Indeed, cross breeds are often healthier and it's a great way of reducing breed related health issues (Frenchies, Pugs and German Shepherds, for example, could do with some major out-breeding in my opinion).
For many pedigrees, demand outstrips supply (hence long waiting lists).
So, simply telling everyone to go via the Kennel Club isn't good advice, as it's not always going to be an option.
Unfortunately puppy farmers do use sites like Pets4Homes, so buyers need to do their due diligence; grill the breeder, see mum with pups etc.
If a breeder isn't eager to share information and answer every Q, I'd walk away.