I am a fan of home slaughter (we home kill all our own meat) so don't agree with some of the more negative comments on the practice in general.
However,
In this particular case. I think the legal position is dubious but more importantly, it's is your land and if you don't like it then you can say no. At the end of the day, they are renting your grazing ie your grass, rather than the right to use your land for their own purposes.