Susanne
A pet trading licence need have nothing to do with standards. The only concern here would be to reduce the amount of unwanted animals. And it need not cost a lot to administer.
Anyone that sells animals for whatever purpose is deemed to be making money from it. And to be making money, you need to be registered for VAT. If it became illegal to advertise a pet for sale without a VAT number, back street trade stops. If it became illegal to buy a pet without being able to supply the VAT number (or registered charity number) of the supplyer, the pet could be removed. If it was found that a companion pet had been sold without making provision for it's neutering, the VAT number could be removed. Trade stops. And if the only reason to have an unneutered animal is to breed from it, you'd need to be registered for VAT. Unwanted breeding stops.
Many charities only let kittens go if people either pre-pay for neutering, or give vouchers to ensure kittens are neutered later. By using the VAT regs, it could be made simple to enforce this outside the charity sphere.
The downside would be that rescues would be swamped with folks who let their queen have an unwanted litter and suddenly found they couldn't home them. But that would only happen once. After that, every kitten would be a wanted kitten. And wouldn't that be nice....
I can do politics, me!