It's not just the plug. When the EU brings in new rules for low-energy washing machines, environmentally-friendly fridges, or whatever, we will obey them without having participated in the framing. Like Norway.
Much more relevant, since it is bigger than the whole of our manufacturing industry, is Financial Services. At the moment, being one of the biggest and most prosperous members of the EU, and having the largest Financial sector, we have a substantial voice in setting the regulations. For example we have blocked a Financial Transactions tax. As a foreign country outside the EU, it will always be that bit easier for EU traders to deal with other EU companies.
Consider Switzerland, which had to agree to obey EU banking laws, even for transactions within its own borders.
People like Davey Grouch try to gloss over the fact that the EU is Britain's biggest trading partner, so we want to accommodate them. UK is not the rEU's biggest trading partner. Renault could halve its trade with the UK and hardly notice the difference.