It sounds to me as though the breeder is rather ignorant rather than lying. £300 is incredibly cheap even for an unregistered pup so she is certainly not breeding for money. Maybe she thinks 'papers' mean the pedigree which is done by the breeder. If the owner of the stud dog is doing them it probably means the bitch's owner doesnt know how to and may genuinely think you can register the pup if you want to. 6 weeks is rather young but if the pups are not going to be socialised and others are going at 6 weeks I wouldnt want to leave one behind. I wouldnt walk away, I would accept that you are getting an unregistered puppy from a mother with a good temperament and a house reared pup that is used to other dogs and children. If you think £300 is expensive you certainly wont be buying a pup from someone else from the more normal £600 plus so I would certainly risk it. My unregistered pup cost more than that and I couldnt afford a registered one so was very pleased to find a sensibly priced litter.