This must be a much more critical issue in the UK than in the US.

We Americans got many of our songs from the Old World and we've always sung 'em however we liked, consciously or unconsciously bringing new and different accents and pronunciations into play. Most of us still feel that we're allowed to sing any damn thing we like, in any manner that suits us, and we generally allow each other the same latitiude.

As I've said many times before, some styles (or genres or subgenres or whatever) of song seem to require their own appropriate "accents" or basic pronunciation styles. I don't think it's "phony" or even irritating for a non-native singer to take a shot at a song whose source is outside the singer's personal experience, and to make an effort to approximate the song's native pronunciation style ~ I'll make my own judgement about how successfully the individual interprets the song.

While there are many differences, some of them quite obvious, among the accents of black and white southerners and among residents of different localities, there is a more-or-less common "blues accent" whose most basic characteristics are employed ~ often quite well ~ by singers of various races and nationalities, including plenty of white guys from England.

I don't object to Mick Jagger, Robert Plant, Eric Clapton, etc., employing an "accent" that is completely foreign to their everyday speaking voices, and that is not a totally authentic imitation of every detail of any particular accent of a specific locale in the American South. Their vocals, like those of any reasonably competent white blues singer, are (generally) appropriate to the material, as long as they convey honest self-expression more strongly than they betray an effort to imitate some foreign accent.

By the same token, I think it's entirely OK for an American singing Irish, Scots, or English songs to employ some kind of "generic" accent appropriate to the song's origin ~ at least to avoid the most jarring American pronunciations that would sound out-of-context. Just as in the case of the British blues singers cited above, an American (or any non-native) should be able to sing ballads from any Celtic or British tradition in a voice that combines his/her natural self-expression with a basic sense of the vowel and consonant sounds that are appropriate to the material.

I'm reminded of a fellow I head live on the radio last week, a Balinese (!) blues artist passing through New Orleans. He was a terrific guitar player and owner of a vintage steel-body National; he performed a few Robert Johnson classics, and the playing was excellent while the vocals were, well, strange. He hit the notes perfectly well, but his pronunciation ~ very "King's-English" British in style, although coming from the mouth of a person native to a former Dutch colony ~ was so jarring, seemed so out of context, that it really detracted from my enjoyment of the song. A little bit of skillful putting-on of an accent would have been welcome.