Two islands, two calculators in your head
Cross-border freelancers lose money in the gap between “I think I can charge interest” and “I can show the number”. UK/NI-style commercial late payment is commonly modelled off Bank of England base + 8% with fixed compensation bands. ROI is commonly modelled off an ECB reference + 8 percentage points without the same UK fixed-band story.
Currency ≠ law
You can issue a euro invoice under UK terms, or a GBP invoice for an Irish-governed contract, depending on what you agreed. The dangerous move is mixing rules because the client’s logo is in London or Dublin. Pick the contract’s governing framework first, then the currency on the PDF.
Practical workflow
1) Confirm jurisdiction on the SOW. 2) Invoice in the agreed currency. 3) When late, calculate under that jurisdiction’s rate basis. 4) Keep the email short and factual. 5) Stop automated chase the second they pay.