Because of the TCA: Goods traded between the UK and the EU are eligible for the preferential tariff treatment. Paperwork proving eligibility is mandatory. Because the UK is leaving the European Customs Union: Goods brought into the UK from countries with which the UK is trading under no special agreement will demand the importer to pay a lower tariff on average. Because the UK is leaving the European Single Market: Goods sent to a EU country from the UK will need to abide by EU standards, which may now diverge from UK standards.