You can streamline the process of exporting under the EU preferential trade agreements by becoming an approved exporter with HMRC.
An approved exporter has an authorisation number which is added as part of an invoice declaration to relevant exports. An invoice declaration will enable the same benefits without any cost.
You can become an approved exporter by completing the C1454 form, available here:
Note that the application process normally involves a full customs audit.
For more information about invoice declarations and the benefits of becoming an approved exporter I would read section 5 of Customs Public Notice 827:
However it is also worth noting that by previously raising an EUR1 and exporting under preference you have already declared you have read and understood this notice.