I tend to believe that "Hun" was picked up from the Kaiser's speech. Ethnic caricature was a staple part of popular entertainment such as the music halls and world events provided grist for their mills just as they do today for comedians. Additionally, the British operated a very succesful propaganda organization which capitalized very early on depicting the Germans as barbarians from out of the east. Take a look at any of the political cartoons and posters of the day. Kipling, who was very involved in Britain's propaganda campaigns, wrote the poem/hymn "For All We Have and Are" in 1917. It begins: "FOR all we have and are, For all our children’s fate, Stand up and meet the war. The Hun is at the gate!..." Haversack.