The pro EU parties who want us to join the Customs Union do not seem to understand what a Customs Union does. The EU one would put up UK prices, harm UK business competitiveness and undermine our flourishing trade with the rest of the world .
If we joined we would need to put tariffs back on a wide range of goods from non EU. We took many tariffs off when we left. We took all tariffs off goods we cannot make or grow for ourselves. Why pay more for them with no home industry to try to protect? We got rid of tariffs on imports needed by UK business as raw materials and components. Tariffs on those raise UK business costs and make our finished goods dearer.It would be self harm to reverse our tariff removals.
If we joined we could no longer negotiate our own Free Trade deals with the rest of the world. We might have to tear up our successful and important trade deals with the Trans Pacific Partnership, with India, Australia, New Zealand, Caribbean countries and others. This would damage trade and relations with important and growing markets.
We would lose our right to speak and vote as a full member of the World Trade Organisation which regulates world trade, as the EU handles that brief and would make us follow their tariff decisions.
The purpose of the high EU tariffs is to protect farms and factories on the continent offering product that would not be competitive without a high tariff on the global competitors. The UK is primarily an importer of goods and food from the EU, not an exporter to them. It is not in the UK interest to have these suppliers protected by high tariffs and charging high prices.
The UK mainly sells services which are not subject to tariffs in most places . The UK needs to be able to sign good trade deals with strong clauses on services. The EU did not help us with that when we were members.