Until 11pm on 31 December 2020, when the post-Brexit transition period ended, the UK was part of the EU customs union. Countries within the EU customs union have no customs duty barriers between them, this is referred to as the free movement of goods.
In order for goods to enter the customs union from third countries and enter free circulation they must be declared to customs and the relevant customs duty and import VAT paid. The customs union members have a common customs duty tariff against goods from outside the customs territory of the Union. This is to protect Union traders against foreign competition as well as being a producer of tax revenue.