If you have an accident involving livestock is doesn't only involve the police DEFRA are informed. I am a firm believer if you want to do this properly then obey the rules. They are there for the animals welfare !!!!
A livestock transport certificate does not test your competency to drive a trailer it gives you permission to carry livestock. A trailer test is about competency but not towing livestock.
Then there are the individual tests you can take for each type of livestock including chickens.
Is it about animal welfare? I seriously doubt it. There are massive issues in animal transport welfare with the long journeys between continents and countries that need addressing before most other livestock journeys, especially those made by smallholders.
Yes, there are rules and like most rules people will decide if they are going to obey them or not. Those people I know who didn't take the individual animal tests did so because they felt their activity was not for economic gain.
If it was about animal welfare it wouldn't matter on the journey length, whether it is for a business or not. Most animals are stressed not by the travelling once in the trailer but the incompetent loading.