If the UK leaves the EU without a deal, any UK business that exports goods to the EU will be responsible for making customs declarations (as is the case for businesses currently exporting goods outside the EU).
If you have clients that trade with the EU, you should check that they have a UK Economic Operator Registration and Identification (EORI) number. If they have not applied, it is important that they apply for an EORI number as soon as possible. If the UK leaves the EU without a deal, they will need an EORI number to move goods into and out of the UK. This identification number will be required even if your client appoints a customs agent to assist in making customs declarations.
An EORI application is made to HMRC and usually takes up to 3 days to be processed. UK EORI numbers start with the letters ‘GB’. Most are then followed by a 12-digit number based on the businesses VAT number. There is an EORI checker available to check EU-wide numbers on the Europa website.