They're not fried in tallow in the UK.
US:
French Fries
Ingredients: Potatoes, Vegetable Oil (canola Oil, Corn Oil, Soybean Oil, Hydrogenated Soybean Oil, Natural Beef Flavor [wheat And Milk Derivatives]), Dextrose, Sodium Acid Pyrophosphate (maintain Color), Salt. natural Beef Flavor Contains Hydrolyzed Wheat And Hydrolyzed Milk As Starting Ingredients.
Contains: Wheat, Milk.
https://www.mcdonalds.com/us/en-us/product/small-french-fries.html#accordion-c921f9207b-item-283bee7dbd
UK (picrel):
McDonald's UK French Fries are made by McCain's using Russet Burbank, Pentland Dell and Shepody potatoes, which are chosen for their quality, taste and long shape when cut. The fries are not coated in any fats or substances from an animal. Once at the restaurant, our fries are simply cooked in dedicated frying vats in a non-hydrogenated blend of sunflower and rapeseed oil which is 100 percent suitable for vegans.
https://www.mcdonalds.com/gb/en-gb/help/faq/are-your-fries-suitable-for-vegans.html