When customs want to tax a commodity they have to know the country of origin, as different taxes apply - depending on the country of origin. That is easy when the commodity consist of one substance. When it consists of many - as with many processed foods, the determination of country of origin gets complex. Is the sugar used in the chocolate, being exported to EU from Britain, from here or abroad? How much of the flour in that loaf comes from Canada - if it is more than a particular percentage, then the country of origin may change..and so it goes on etc etc.
If we do not agree a comprehensive trade deal with the EU, 'rules of origin' could play havoc with movement of food in and out of the EU. "Rules of origin are the complex requirements that determine whether or not a product is produced 'locally' in the UK or the EU – its economic nationality. If it is not deemed to be sufficiently British, it may not qualify for preferential tariff rates."
The ingredients of many UK food products are a rich mix of goods from the UK and around the world, many of which are not produced in the UK or not in sufficient quantity throughout the year. The levels of global content that will be allowed in 'local' food and drink products will be set during future UK-EU trading negotiations. There are 15,000 Process Agricultural products with various tariffs. But nobody really knows what/how this will get sorted. Mind you David Davis' previous experience of working at Tate & Lyle may come in handy with this one...UK chocolate producers (who export £530m of products each year to the EU) could face tariffs of 27+% depending on the value of UK refined cane sugar originating from the world’s poorest countries, plus all the Irish milk in their products. Most milling flour could well be hit, as most have a mixture of British flour with N American flour. Food Manufacturers could face the prospect of either a costly restructuring of their supply chains or de factor barring from future EU-UK trade as a result of the EU’s Most Favoured Nation (MFN) tariffs which are prohibitively high for food and drink. Or we could produce a lot more food ourselves!