From 1 January 2021, the UK will operate a full external border with the EU.
This entails major change for imports and exports to and from the EU. From 1 January 2021, declarations will be needed to import or export specific (limited) goods categorised as 'controlled': see here for imports and here for exports. But for non-controlled goods brought from the EU to GB, import controls apply in three stages: January, April and July 2021. Some changes will apply to all goods movements, and will involve customs declarations, customs duties and VAT on imports, and safety and security declarations. 'Additional requirements' come in, but affecting only certain specific goods movements, such as foodstuffs. Note that separate rules will govern goods moving into, out of and through Northern Ireland: this link may be useful in this case.
Economic Operators Registration and Identification (EORI) numbers: from 1 January 2021, an EORI number with the prefix 'GB' is needed to move goods between the UK and the EU, unless you only move goods between Northern Ireland and Ireland - see here for how to get an EORI number.
Remember that from January 2021, it will be important to think about both the UK and EU sides of the equation: to comply with EU requirements, you will, for example, need an EU EORI number if your business makes customs declarations or gets a customs decision in the EU.
Using a customs intermediary: given the complexity of UK and EU customs declarations, you may want to engage a customs intermediary to deal on your behalf.
Postponed VAT accounting for goods imported from the EU: from 1 January 2021, import VAT applies to imports from the EU. Using 'postponed VAT accounting' from 1 January 2021 lets you account for import VAT on your VAT return, giving the potential to declare and recover import VAT on the same return - see the information provided here.
Delaying customs declarations and payment of tariffs: when the UK's full suite of border controls are in place in July 2021, full customs declarations and payment of customs duties, as set out in the new UK Global Tariff (or as specified in any trade deal with the EU) must take place when goods are imported from the EU. But from 1 January 2021 to 30 June 2021, most traders with a good compliance record can defer declaration and payment for up to six months on imports of standard goods from the EU. Information on delaying declarations can be found here.
This is only a summary outline of some of the issues involved. Gov.uk provides an online checker tool to use in your own circumstances. Do talk to us where further advice is needed.