After Brexit, the UK will have to leave the EU Customs Union (CU) and become a legally separate customs territory. It might then, however, seek to create a new Customs Union with the EU to cover their mutual trade.
The UK Trade Policy Observatory at the University of Sussex (UKTPO) has produced a short animated video that explains what this entails, and what kind of an agreement the UK and the EU would need to establish in order to achieve the same level of trade costs as we have now. Ultimately, the video explains, a Customs Union will not produce ‘frictionless’ trade without re-creating several aspects of current EU membership. With Brexit negotiations already underway, it emphasises that maintaining a customs union is just one part of the story; and not, by itself, the be all and end all for achieving a smooth trading process.