[stagbeetle]

For the British English claim, I have no source nor can I find one. It's what I was taught long ago and that the Fowler brothers use it implicitly in all of their series.

However, I do have a rebuttal to your style guides. They are only that: descriptivist style guides. They clash with one another and are nothing more than another dialect of English. Whereas Loweth's English Grammar, the recognized beginners in English prescriptivism standard, does not recognize proper adjectives.