I'm not avoiding any points on campylobacter and chickens - Europe focuses on controlling pathogens such as campylobacter on the farm, rather than controlling it later in the chain, as in the US. Chlorine washing only addresses surface contamination and not contamination within the meat, which is what the European approach is aiming to address.
Yes, some UK chicken at the point of sale is still contaminated with campylobacter - but then so is some US chicken, so chlorine washing doesn't work either. Not a lot of difference in the risk - just one chicken hasn't been washed in chemicals.
And you do realise that the legislation like the Nitrates Directive is aimed at preventing surface water and groundwater pollution, given that these are our drinking water sources? Nitrate pollution has increased massively since WWI due to agricultural practices, and will continue to be an issue in groundwater for decades to come given how slowly nitrates migrate from the unsaturated to the saturated zone. So even though the use of nitrates has been controlled since 1991 it will be many years before peak levels are reached - hence don't expect controls to be rolled back just because we're leaving the EU.
Why does it matter that those farms aren't selling into the EU? In the case of nitrates those farms impact on public water supplies in exactly the same way as farms that do export to the EU.
And:
No, I don't buy pre-washed salad.
Yes, I am selective about which meat I buy. If the US ends up importing chicken to the UK I wouldn't buy it, but I expect millions would, especially if it was cheaper than UK produced meat. That would be their choice.