The English have acquired a tainted reputation due to centuries of colonial dominance and exploitation. Especially in the second half of the twentieth century, with the growth of nationalistic sentiments across the Commonwealth, anti-English sentiments increased. It has been said that the smaller home nations are jealous of English political and economic success; this argument surely has some relevance. One must not ignore the fact, however, that the English are often culturally ignorant of other areas' local specificities (very much in the way Americans stereotypically are).
Calling someone an 'Englisher' firmly places the caller in opposition to the English claim to cultural superiority.
Don't be an Englisher now, come on!