From the New York Times
It’s a different bar from the Cadbury bar available in the United States. According to the label, a British Cadbury Dairy Milk bar contains milk, sugar, cocoa mass, cocoa butter, vegetable fat and emulsifiers. The version made by the Hershey Company, which holds the license from Cadbury-Schweppes to produce the candy in the United States under the British company’s direction, starts its ingredient list with sugar. It lists lactose and the emulsifier soy lecithin, which keeps the cocoa butter from separating from the cocoa. The American product also lists “natural and artificial flavorings.”
Tony Bilsborough, a spokesman for Cadbury-Schweppes in Britain, said his company ships its specially formulated chocolate crumb — a mash of dried milk and chocolate to which cocoa butter will be added later — to Hershey, Pa. What happens next accounts for the differences.
“I imagine it’s down to the final processing and the blending,” he said. After consulting with chocolate manufacturers in each country, Cadbury tries to replicate the taste people grew up with, he said. In the United States, that means a bar that is more akin to a Hershey bar, which to many British palates tastes sour.