Richard Murphy, has been arguing that the tax break is what has pushed up the land values because it has turned farmland into a finacialised asset used precisely to avoid inheritance tax
https://www.youtube.com/watch?v=s9J0GpnXNhY
I have no ability to assess the argument. It seems plausible, but more plausible still is the idea that land values are high in the hope that the land will be rezoned and available for housing. Speculation as much as anything else