I can't get onto the Hershey site, but if the nutritional information is correct on a dieting website I found, then all things considered, the only big difference between a Hershey bar and a Diary Milk Bar is the amount of salt. Hershey's has more.
The rest is basically the same in terms of amounts, sugar wise, fat content wise etc.
Now, of course they use different types of fat and Hershey scalds the milk, which accounts for a sweeter taste, but Hershey does not use a crazy amount more sugar, in fact it uses slightly less (56.7 grams of sugar in DM per 100 gram bar compared to 53 grams of sugar in 105gram bar of Hersheys).
So the general compostion of the bars are the same, but of course the actual fat or whatever they use is different.
But of course they aren't making Dairy Milk for me, nor is Hershey making chocolate for you. They are making it for whatever sells the best in whatever country they are selling it in. So Cardbury's is different in every country in the world, unless you want to pay import prices.