In some cases you will be eligible for the 'zero-tariff' (no import duties) negotiated during the Brexit-deal between the EU and the UK. By providing a proof of origin you declare the country of origin of your goods in the commercial or pro forma invoice. This is true for both shipments to consumers and businesses. For this proof of origin you need a UK EORI number.
It is not possible for DHL to adjust already submitted customs documentation in case you wish to add a proof of origin later. You will receive a confirmation from DHL which you can use to request a change to your declaration yourself at the Dutch customs authorities.