If I were to guess, the primary difference (and I looked at the ingredient listing US to UK) is that the US Oreos contain high fructose corn syrup (as so many US products do), whereas the UK Oreos do not (glucose & fructose syrup, yes, but not HFCS). That might explain why the US ones degrade more readily in milk?
On the odd occasion when I buy Oreos here, I think they taste fine. A friend at work had some Double Stuff ones recently that she brought back from the US & my gut hurt right after eating those.