What I do know is that in the UK it is illegal to feed live prey to a pet. (I wonder if this implies invertabrates as well, or just rodents)
I was unaware that people were getting charged with euthanasia of rodents BUT, here is something that may be interesting to everyone here...
In many countries, the UK, USA, and Canada among them, it is illegal to euthanize or otherwise practice veterinary medicine without a DVM liscence or adequate training and supervision from a DVM (a vet tech would be an example). Contact the AVMA, CVMA, or British version of such if you'd like verification of my statement. The word euthanize is actually euthanitize, and is latin for humane death. Being a veterinarian gives you the background, expertise, and proffesionalism to legally judge what is humane.
Now, are all rodent breeders under veterinary supervision? Perhaps some, but the vast majority probably not. Do I foresee a crack down on this practice anytime soon? No. Will you be convicted on animal cruelty if found to not do it properly? Yes.
The UK is much more stringent on animal cruelty laws than in North America, which is a little ironic in my opinion, as their veterinary practice is in many points far behind what occurs in Canada and the USA.