I don't know if it helps, but I'm not keen on being referred to as a "Brit" and I don't like being referred to as a "Limey" at all.
Germans I've met find "Kraut" offensive; but I don't know how representative of Germans as a whole that is. Chinese people definitely find "Chinks" offensive, and those from Pakistan without doubt find their particular abbreviation offensive.
Dunno. Do Americans find "Septics" offensive?
In your examples, there's differentiation.
Brit is a shortening of Brittish or Brittain. So is Sov or Jap.
Kraut refers to sauer kraut, alluding to a cultural food.
I'd have to look up what Limey referenced.
I have no clue how Septic would relate to America.
But there's a world of difference in a shortening a nation name and using a typically deragotory slang.
Culturally, americans shorten up names of people and places all the time. Recipients may be offended, but that's just how we speak.
One should be offended by the time we rounded up those people and put them in a camp, or lit their house on fire and drove them out of town, or blamed them for the lack of jobs, or the bad economy. Not by a simple shortening of a name. One is a matter of right and wrong, the other is a matter of wounded pride over an imagined slight.
Choose which one to war over.