From 1 January 2022 the rules for selling to, and buying from, the EU will change.
HMRC have issued information about these changes.
Broadly, from 1 January 2022, businesses will no longer be able to delay making import customs declarations under the Staged Customs Controls rules that have applied during 2021. Most businesses will have to make declarations and pay relevant tariffs at the point of import. However, see details of Postponed Accounting.
Please also see a publication issued by the Cabinet Office which includes a Policy Paper on The Border Operating Model.