The UK HM Revenue and Customs (HMRC) explains changes to customs, VAT, and excise to the UK VAT-registered businesses only trading with the EU, in case of the UK leaving the EU without a deal.
For the HMRC documents, please see here:
Letter to EU-only traders in the UK (except Northern Ireland) – December 2018
Letter to EU-only_traders_in_Northern_Ireland_-_December_2018
Detailed guide: Exporting and importing goods if the UK leaves the EU with no deal
Detailed guide: Moving goods to and from the EU through roll on roll off locations including Eurotunnel
Detailed guide: Declaring your goods at customs if the UK leaves the EU with no deal
Detailed guide: Customs procedures if the UK leaves the EU with no deal
Detailed guide: Get a UK EORI number to trade within the EU