Perhaps you could say IJN, or IJA, rather than Jap.
"Brit" isn't quite on the same level as "Jap", because British is a nationality - a nation of nations - whereas Japanese is both a nationality and an ethnicity. Jap isn't the kind of thing you would use in polite conversation, and there's no point trying to rationalise that, because language and society are not rational. Rationalising slur words is like trying to impose the Queensberry Rules on a war; nice idea, wrong moggy.
Speaking as a British person, "Brit" is incredibly mild, milder than "Yank" or "Ocker", considerably milder than "Kraut". It's odd that my ancestors had so many words for the Jerries, but the sausage-eaters just had "Tommy" or "Englander schweinhund" in return. "Brit" sounds like the kind of thing an American might say if he wanted to give the impression that he was friends with a British person.
In the context of aircraft, most ethnicities or national terms are shaky because the planes were flown by so many national air forces; I'm sure a Finnish 109 pilot would be miffed if you called him a "Kraut". The IJN/IJA planes are ironically one of the few exceptions to this, as they were AFAIK not flown by non-Japanese forces during WW2 or, if they were, it was very obscure.
(ps I originally referenced "Diamonds are Forever", the bit when Bond shoots the cat, and Blofed says "Right idea, Mr Bond, but wrong cat", except that he doesn't say cat; amusingly, and understandably, it was turned into asterisks. At least in the UK the rhymes-with-plussy word has a tiny, tiny vestige of its former meaning)
