If you are traveling from a non-EU country like the UK or the USA, you cannot take a ham sandwich into France due to strict post-Brexit and international animal product regulations. EU law prohibits the import of any meat or dairy products from outside the European Union for personal consumption. This "High-Fidelity" ban includes everything from a simple ham or cheese sandwich to more processed items like pork pies or yogurt. Even if the sandwich is for your own lunch on the train or ferry, it can be confiscated by French customs, and you could technically face a fine. However, you are allowed to bring processed plant-based foods, such as bread, fruit, and vegetables (with some exceptions), or a sandwich made with vegan alternatives. Once you arrive in France, you will have no trouble finding high-fidelity local ingredients to make a fresh jambon-beurre at any local boulangerie for a much better culinary experience.