Strictly there isnt a law against allowing ragwort to grow BUT there is now a code of practice , the breaching of which can be used as evidence in the neglect and abuse cases brought by the likes of RSPCA.
So the code itself isnt statutory but failing to adhere to it can be contributory evidence of the wider offence. Like the 'five freedoms' it sets out what a horse should be expected to be provided for, and in this case, protected from.
In practice the stuff is everywhere, our place it had been left to run riot for 30 years before we came, so its an ongoing fight, as well as hours and hours of pulling it we are having 5 fields ploughed and reseeded as part of the battle. (Spraying is tricky with ours and nighbours well water supply close by).