What’s the difference between a Single Market and a Customs Union? Goods imported into this country pay an additional tax known as a tariff. A Customs Union is where two countries agree to charge a single rate of tariff against imports from any third country. A Single Market is where two countries agree not to charge tariffs on imports from each other.
It is possible to have a Customs Union without a Single Market. Two countries can have a common external tariff, but also charge each other, presumably at a lower rate than the external tariff, since otherwise exporters would just send goods to a tariff-free area in a third country before importing to the other country. Such tariff-free areas do exist.
A Single Market without a Customs Union would mean importers bringing in goods through the country with the lower external tariff, whence no income for one of the countries. This is not likely to endure.
At the moment, a tariff needs to be paid by many businesses if they want, for example, to move goods from Stanwix to Kingstown. It is called Value Added Tax, its original rate was 8%, and it has risen to 20% without appearing in any manifesto at a General Election. This tariff was introduced when we joined the European Union and its existence and history are good reasons to vote to leave it. At the time we joined, the European Union was known as the Common Market, but really it has always been more of an anti-market thanks to VAT.