From my very brief reading of the European Union (Withdrawal) Act 2018 all EU laws would continue to apply after Brexit day but they would no longer be regarded as EU laws but as purely UK laws meaning that: any NEW rulings or amendments from the EU would no longer apply in the UK, the British courts would have the final say on what the law means rather than the ECJ etc.  As a result overtime UK and EU would diverge. The transition period is an opportunity to manage this divergence.

If you are importing goods then, I suspect that the UK will, initially allow things to enter from the EU even though there is no framework to manage this as they don’t won’t to shot themselves in the foot too much by applying border controls. This includes not just actual goods but recognising EU lorry drivers etc and allowing them through on the nod.  If you are exporting stuff TO the EU, that is harder to judge. There will come a point where the EU will say we now must impose our standards on goods and anything that does not comply with our regulations not is allowed in. I have no idea when that will be (nor does anybody else).

https://www.gov.uk/government/publications/regulating-chemicals-reach-if-theres-no-brexit-deal

Basically carry on - the existing enactments will be fixed as UK law BUT actvity where communication is with ECHA e.g. registration will transfer to a UK authority

HSE has a web page http://www.hse.gov.uk/brexit/brexit-chemical-regulation.htm

For those who know their role under REACH there is a summary table

http://www.hse.gov.uk/brexit/ag-scenario-table.pdf