U.K. has formally confirmed to the EU that no extension will be required, and none will be accepted in case the EU requests one. That sets the Brexit date invariably for 31.12.2020
Customs Clearance will be required for Imports and Exports after 31st December regardless of whether a FTA with EU has been agreed. HMRC will release the UK Global Tariff which will apply to all imports from 1st January 2021, however some duty may benefit from preferential rates subject to UK and EU signing a Free Trade Agreement.
Import/Export entries will also be required between Great Britain and Northern Ireland. HMRC cannot introduce unilateral duty facilities in NI as it will be working to the EU Customs code, although NI remains part of the United Kingdom territory.
We strongly recommend you issue a valid UK EORI number for imports and exports to/from the United Kingdom.
For help with your commodity codes pls access: https://www.gov.uk/check-tariffs-1-january-2021
If any doubts, please contact your local CGATE office.