There are no cases that have ever been reported in the UK where poisoning to human beings has been found to have been caused by ragwort. Furthermore the research is clear that it is very unlikely to ever occur. Ragwort tastes so bad that animals are repelled by it. The amount of ragwort that would need to be consumed by a person to damage them would be enormous. It is only mildly poisonous and there is no serious risk of liver damage from handling the plant, from its pollen or from being contact with it in any way.
There are mild toxins present which can be absorbed in minute amounts through the skin but these do not pose any significant risk to the public. . There are many many more dangerous substances present in other plants and alcohol consumption is a far bigger risk to the livers of the general population.There are many scare stories around but they are generally without any substance or foundation.
The one exception is the allergy that causes dermatitis that Ragwort can cause, in common with many daisy family plants. This is caused by different substances and poses no risk to the liver.
For more detailed technical information and a fuller explanation of the research please see this page. Ragwort humans
There is also a simple fuller non-technical explanation here Ragwort poisoning Humans
For information on ragwort and the law. See The Weeds Act 1959 and in the Ragwort Control Act 2003.