As part of the single European market, EU member states have worked to ensure satisfactory product and safety standards. Under the umbrella of the CE Marking Directive many product standards and European norms have been created and brought into British law via British Standards (BS). If you want to supply the UK, or any other EU country, your products have to comply with the relevant standards and display the CE mark.
The UK, being in the EU, has a part to play in shaping these product standards, norms, and directives to which we must then comply. If we leave the EU - but still wanted to trade with the single market - we will still have to comply with all relevant regulations but would have no further hand in shaping them.