The full impact of Brexit will depend a lot on the negotiations that will happen, and until the negotiations are finalized and implemented the EU’s environmental and chemical laws will apply. If the UK makes the decision to stay in the single market through membership of the European Economic Arena, it will continue to be required to implement REACH, but they would lose the ability to take part in the decision making process.
Another possibility, after the negotiation process, is that the UK decides to opt out of the single market. If the UK decides to do this, REACH regulation would no longer apply for UK based companies. This would mean that the companies would have to get their own importers or appointed only representatives in the EU to register their substances. For now, everything remains the same until a decision is reached.