I used to live quite close to Bourneville (Cadbury Central), and I can totally tell the difference between English and American Cadbury. I can even tell the difference between Canadian/English/American Cadbury!
The Easter chocolates come closest in taste to English Cadbury, especially the Creme Eggs and Mini Eggs. For the regular bars, like Dairy Milk, you're just paying double the price for a Hershy bar.
BTW, if you're ever in the Midlands, I'd definitely pay a visit to Bourneville's Cadbury Chocolate world. They practically force feed you the entire tour, and when I was there, they had a chocolate Coronation Street!