The situation is complicated. As I understand it the USA product is "manufactured" (perhaps thrown together might be a better term) under licence by Hershey, and is different to the UK's. The flavour has changed over the years, unfortunately, partly due to different manufacturing processes, and indeed the UK-made chocolate did often taste different to that made abroad. Talking about Cadburys Dairy milk (CDM), at the time that I worked there (including about 2 years when I routinely tasted the daily production sample at Bournville) the cho colate sold in the UK came from two sources, the majority from chocolate crumb produced at factories (Marlbrook, Frampton) in the UK using a continuous process, and some from Ireland, who used crumb from the Irish crumb factory, which had a batch process. To my mind (and many others) the Irish product was much nicer . the Irish factory mainly concentrated on certain products. As an aside, pre around the late 1970s (th date is a bit of a guess) a few lines used miklk powder chocolate (and was never claimed to be Dairy milk), as I remember mostly for some coated cheaper lines and all manufactured at Somerdale and the product was not as good - similar to the not very nice product from Rowntrees. But then it was decided that all their chocolate should be of CDM quality and the recipes were changed. At around the time of the takeover by the artificial cheese makers it was decided that for a limited number of coated lines an outside supplier should provide the chocolate . To be honest, I am doubtful FOR COATED LINES WITH A STRONG FLAVOURED CENTRE whether more than about 1% of the consumers would notice the difference, so can understand why this was done as CDM is appreciably more expensive to produce. The outside firm supplying the chocolate freely admitted ( at that time) that, though they thought they could match the flavour any other milk chocolate on the market, they were unable to copy CDM. Since then I gather the Americans have lntroduced non-CDM chocolate into some other lines . I Have no knowledge of changes since the takeover. they may have removed the flavouring oils (which are included with vanilla flavouring as "Flavouring") and may have changed other processes. Some emulsifiers have a distinct flavour (the Cadbur one in the uk at the time I have discussed was flavourless , but had the misfortune to be "artificial"). One should ignore the purist's comments about "palm oil" . Anyone under 55 years old is unlikely to have tasted CDM with no added substitute unless they have tasted chocolate produced specifically for export to places where cocoa butter substitutes (CBE)are not allowed. A good CBE at low levels is not deleterious. Far more deleterious to the eating quality of the chocolate is the reduction in size of bars - a thick chunk has a better texture, mouthfeel and eating quality than some of th ethin efforts now being produced.
Should add finally that I agree that CDM does not taste as good as it once did, though do not know the reason