> America and Australia are, to my ear, in the same boat as the UK: accents abound but if you were to transcribe people's speech they would all appear to be Californian.

While regional variations in mainstream, WASPy, middle-class-and-above speech may be more accents (in the vocal sense rather than the linguistic sense) than dialects, American English has a number of clearly distinct dialects (notably Chicano English and African American Vernacular English) and accents (in the linguistic sense that includes patterns that would be distinct in transcription; including a couple different American Jewish English accents, among others).