Uh, I think joyboytoy79's post was tongue-in-cheek, milkwhite. Everyone knows that the British Isles have been invaded by and occupied by diverse groups over the millennia. Aside from the mix of genes it left, it created a (large, flexible) language that's also an amalgam of many roots.
My personal experience? In Europe, the Dutch overall are very well-hung guys (on average, they're the tallest in Europe, too). Next up are the British and Irish, followed not far behind by the Germans. FWIW, I also find that Northern European guys have longer foreskins than the rest, but I have my theories on why.
Americans don't seem to be anything special in the basket department. In fact, I even remember one good friend from college, while I was busy giving him a blowjob, apologize for "the WASP curse". But he got no complaints from me.
If there is such a curse, it's odd that it's in descendants from Blighty. But who knows? Maybe the Puritans came over because they were being persecuted for their uptight ways in Britain and were self-righteous because they were poorly hung. While big-dicked hedonists went about their usual ways back home.