These are the US ingredients:
INGREDIENTS: SOYBEAN OIL, WATER, WHOLE EGGS AND EGG YOLKS, VINEGAR, SALT, SUGAR, LEMON JUICE, NATURAL FLAVORS, CALCIUM DISODIUM EDTA (USED TO PROTECT QUALITY).
(
http://www.mayo.com/products_mayo.asp#mayo)
These are the UK ingredients:
Vegetable Oil (77%), Water, Pasteurised Egg & Egg Yolk (8%), Spirit Vinegar, Salt, Sugar, Lemon Juice, Mustard Flavouring, Antioxidant (Calcium disodium EDTA), Paprika Extract.
Looks like the main difference is the type of oil used.