Having only just joined I've missed the original question about ragwort but my understanding about it is this. Sorry if I'm repeating what has already been said.
The above site seems to be mudying the waters by not telling the complete truth. This info from Cheshire County Council site was updated this April and gives bit more information.
"By virtue of the Weeds Act 1959 and the Wildlife and Countryside Act 1981, the County Council has a statutory duty to control harmful/noxious weeds on the highway. Hence, the Council will undertake to treat the following harmful weeds: broad leaf dock, creeping thistle, curled dock, spear thistle and ragwort. We also treat the notifiable weeds, giant hogweed and Japanese knotweed. One of the more common harmful weeds is Ragwort, most notable by its distinctive yellow flowers which bloom from July to October. Unfortunately, it can prove fatal to cattle if ingested. It is treated by a combination of targeted applications of weedkiller in the early part of the growing season and removal by hand later in the season".
Contrary to the Ragwortfacts site it does appear that there are notifiable weeds in the UK, its just that Ragwort is not one of them even though local authorities are legally obliged to deal with it on their property. They also say that it doesn't accumulate in the body yet in the next sentence state that the damage IS accumulative. I think thats being a bit picky.
Most livestock only live for a few years at most before becoming part of the food chain so if some ragwort has got into their hay its not likely to do them much damage over such a short time. Nothing will eat it whilst growing but once dried it becomes palitable. Like many poisons, its damage is accumulative so you need to have eaten or been in contact with it over a long period.
The bit on that site about horses being poisoned even when it is not in the field is also misleading. They may be correct about the damage not being done by spores or seeds etc. but they don't mention what would happen if the horse was being fed hay cut from a field in which ragwort was growing. It would take many years and lot of hay with ragwort in it but it can happen.
Hope I haven't muddied the water any more.