But what's to stop Bob down the pup ignoring the law?
The 'breeder' is someone making a quick quid off some bitch he owns. He doesn't give a rat's arse about the law - he's just going to sell the puppies for £50 to make some quick cash.
I'd suggest your example above makes it MORE likely puppies are sold down an alleyway because it makes it harder for people to get hold of dogs from even semi-ok breeders. So they turn to the bloke down the pup and buy his puppies, no questions asked, no laws abided.
How will we ever catch them?
Ask vet's to check? Then you risk people refusing to get medical care for their dogs when it's needed, in case their license status is checked.
Ask the police to do random stops? They already cannot keep up with the workload and have only lately been wanring that professional criminals now have real control of the country.
How else are we going to check licence status of people who bought a dog cheap and without checks and who avoid authority? Because they are the people we need to target the most...