Mole said: “The emerging gap between UK and EU pesticide standards is incredibly concerning for our human health and environmental protections, but also for the future of UK agriculture as our standards fall further and further behind those of our largest trading partner. UK food exports containing pesticides that EU growers aren’t allowed to use, are likely to be rejected. Given that the EU still accounts for around 60% of UK agricultural exports, the impact on farmers could be devastating.”

I'd think that the same issue is going to come up for wherever a farmer exports to. Some countries are going to not want given pesticides used, some don't like genetically-engineered foods, some approve specific genetically-engineered foods, some don't allow certain foods from certain sources only (e.g. I know in the US we had many years of a ban on importing British beef after the mad cow disease outbreak). Unless the UK and that other country have exactly the same set of requirements, a farmer is always going to have to have the food conform to those standards.

Yeah, it's simpler for British farmers if the UK also has the same restrictions, countrywide, as a given export market, but I don't think that it's a fundamental issue for exports. It does mean that they need to ensure that they avoid violating standards for that export market in addition to domestic requirements.

Here's a graph of US exports:

https://www.ers.usda.gov/data-products/chart-gallery/gallery/chart-detail/?chartId=58374

The top five markets for US agricultural exports are China, Mexico, Canada, Japan, and the EU-27. While, under the USMCA FTA, Canada and Mexico do have mutual recognition of standards, so that the onus of proof in disallowing a comparable standard from another USMCA member is on the rejecting country, normally things acceptable in one country are also acceptable in the others, and that doesn't require that their standards are identical. Japan and China and the EU-27 definitely don't have the same standards. Just means that stuff that is exported to a given destination needs to conform to whatever requirements are placed in that export destination, even if stuff that isn't going there doesn't have that requirement.