Are there any constants that we actually know to have varied along the lifetime of the universe?

Because there obviously is a demand for the produce and the market will not contact, instead it will supply elsewhere, whether it is cheap and not regulated yet. Probably regulations will catch up, but this is incredibly slow process and first thing will have to get bad. I am in no way saying keep things the same, quiet the oposite, it is not enough and has to happen across the entire european farm industry.

how farmers will compete with foreign products without the same pollution requirements

That’s the big issue. Even if dutch farmers agree to implement costly measures to limit emissions and pollution, unless this is enforced EU wide, this will just drive them out of business and move the pollution problem somewhere else. And I do not really see this happening EU wide in any foreseeable future.