You don't need to be a member of the EU to be accepted to be part of European Chemicals Agency which is the body that governs REACH. A number of Non-EU European countries are part of the REACH approval process. I could be wrong but I think even Taiwan were offered a pay to play deal to use the ECHA data for their equivalent of REACH to reduce total cost for their jurisdiction.
The EU choose not to offer such access to the UK which is well within their rights.