One thing Rees Moog and his crew want from Brexit is a lightening of the regulations around chemicals

I think you need to be a little more specific. The European Union implemented the Globally Harmonized System (GHS) through its Classification, Labeling & Packaging Regulation (CLP) which became fully effective on 1 June 2015.

"Hazardous chemicals in products like toys and pizza boxes are a global threat and must be regulated under an international protocol, NGOs told the European Commission ahead of a UN meeting in Stockholm next month.

A legally binding protocol in supply chains across the world is needed to avoid the presence and improve the tracking of hazardous chemicals in products, said several NGOs in a letter to the European Commission this week.

The letter was sent ahead of a UN meeting where world leaders will discuss a post-2020 policy framework to minimise the adverse impact of chemicals on the environment and human health."

I think you cry wolf. 40 years ago IATA was probably the only body that had a prohibited list of chemicals (Carbide was one of them, as I tried to ship it-very unsuccessfully) Now every tanker on the road boasts a hazmat sticker, even extending to carrying propane cylinders in a van. The trend is to increasing legislation governing labeling,
transporting, selling and utilising anything perceived to be a hazard. The chances of such legislation becoming anything but ever more stringent is very unlikely.
Many of these constraints are the result of international agreement meaning a departure from the EU would have zero impact.