I suppose it depends on the adjective being used. You're right, Bluegiraffe, that "I'm an English" sounds very awkward. However, "I'm a Canadian" or "I'm an African" sound correct to me. Would you describe yourself as "a Brit" or is that an Americanization?
I attribute this to the ambiguity of the word "English".
You can have Canadian nationals but you cannot have Canadian speakers.
You can have African nationals but you cannot have African speakers.
You can have English nationals and you also can have English speakers.
So I will be throwing my hat in with the
crowdit depends on the adjective being used