EU-UK Trade and Cooperation Agreement: returned goods relief vs preferential origin

European Commission released Guidance on preferential treatment, origin and customs procedures to assist EU businesses importing from and exporting to the UK. One of the questions clarified: “Goods originating in the EU are exported to the UK and released for free circulation there. They are not transformed. Can they come back to the EU without paying duties?”. The only suggestion is to customs clear them as returned goods. Does the same principle apply to all EU FTAs? What are the requirements for returned goods to claim “zero duty”? What about “zero VAT”? Other requirements like Sanitary and Phyto-Sanitary?