EUR1 documents are used to obtain preferential duty rates for those countries the UK has a trade agreement with.
In order to claim preference goods must meet a set of origin rules that will be laid out in each individual trade agreement.
Trade agreements can be found on the UK Government website
If you need to certify the origin of your product and potentially benefit from favourable trade terms, a Eur1 Document is needed. The document forms part of documentation required by customs. You will need a Eur1 document if:
If this applies to you, you should be using a Eur1 document.
Since leaving the European Union, the Eur1 document has replaced the EU Eur1. The UK has preferential trade agreements with certain countries which and the Eur1 document will allow preferntial rates to be applied, where applicable.