I forsee a grave problem in that if what Blazey says is true and you can just get the shop to sign something saying it's captive bred or whatever.. how can that be proved? No chippings, no markings, no identifications...
And how does the shop prove that what the wholesaler said was true? How do they prove that what a private breeder or a reseller said was actually true? If there is no governing body identifying animals and issuing paperwork then there is any point down the line where someone can tell a lie, whether purposefully or just because they didn't realise.
So to implement a law requiring paperwork for certain species already well established in the UK is going to just cause havoc surely (and i haven't read the livefood thread.. sorry), because no one can 100% prove anything about the animals they have unless they actually bred them themselves, and somewhere down the line you're not going to be able to prove the parentage.
I hope that if this comes into effect it will only affect newborns and that any existing animals will be issued paperwork without having to prove heritage, because to prove heritage on species that you bought 5, 10, even 20 years ago off some guy in the local pet shop that's now moved on, would be impossible.