Your editor is correct. WWI is an Americanism. I did a PhD in military history, wrote academic articles and taught at the military academy for some years (until history lecturing became an unviable occupation). It is a bit silly saying it is offensive, but he's right that we don't use that abbreviation in the Commonwealth, we also don't use the spelling "defense". The appropriate reference is First World War and Second World War, and of course 'defence'. You may see WWI and WWII written in books designed for the US market but otherwise it is avoided, if you want your writing to be accepted in military history spheres it's better to avoid those abbreviations. Happy for you to PM me if you have any other questions re military history.