By definition part of the rationale behind the European Union project and their long term desire to create a single European superstate means that the EU has to be a single market. So logically, leaving the European Union means not being part of the single market. It does not mean not having access to the single market. It does not mean the U.K. no longer wishes to trade with the EU and their single market. As it is anyone can sell into the single market. No different to selling goods to any other country. It is though common sense that if you wish to sell good to another country you have to comply with their requirements, and so it is with the EU and their single market. Of course the EU may threaten to impose tariffs on goods we sell to them. This is going to be a real test for the EU. Do they really believe in free trade? The U.K. will be saying to the EU we want to have free trade with you. The question is how will the EU respond. As the EU sell more to us than we do to them (not surprising given the respective sizes) if the EU do impose tariffs and we therefore also impose tariffs on imports from the EU it follows EU exporters will be paying much more than UK exporters.
We must leave the EU as the British people voted for. We must leave the Single Market, offer the EU a free trade deal, see how they respond and we then respond accordingly. Meanwhile, we must go as far as we can in putting in place new trade agreements with countries around the world. There are tremendous opportunities across the World for our companies.