Essentially the UK will 'retain the benefits of the Single Market and the Customs Union' up until the 31st of December 2020, for UK traders it's a case of 'as you were'.
Provided the Brexit transition deal is ratified by all EU27 member states, expected in October, UK business will be given an extra 21 months' breathing space. Up until January 2021 UK international traders can retain current practices and workflows with respect to the EU i.e. export and import declarations will not need to be made for goods consigned to the EU; Tariff barriers will not be in place; Goods not in EU free circulation will still need to be moved under the Community Transit system; Excise goods processed through the EMCS system.
The origin of your goods will still conform to the EU rules of Origin, so if you are making use of one of the many EU preference deals or FTAs via the EU based forms, EUR.1, FormA, A.TR these will still be valid, as will the EU Certificate of Origin. We do however recommend seeking
Authorised Exporter (AE) status from HMRC, as this would likely be how the UK will manage FTA's & GSP post-Brexit.