Muckanaghederdauhaulia". There is a longer placename in England: Wiggenhall St. Mary the Virgin (you have to include the "the Virgin" to distinguish it from the nearby Wiggenhall St Mary Magdalen). I guess that this doesn't count as it is more than one word. But shouldn't the section use the phrase "one-word placename"?  

Since the article is titled Longest word in English, not longest phrase, that seems a bit redundant. -  

The article should include the longest placename in England, isn't it Sutton-under-Whitestonecliffe not Wiggehall St Mary the Virgin / Magdalen