EU-UK FTA: What is exporter reference No? (Part II)

We overviewed the EU requirements for an EU exporter to get the Exporter Reference Number to include in the statement on origin (in Part I) and continue answering these questions: What is the Exporter Reference Number of the UK exporter? Who may act as an EU/UK exporter? What is a “commercial document” on which the statement on origin may be made out?