Bob Copper, Yes indeed a great singer and a good honest Sussex accent. But on one occasion when I visted him at his club he sang a blues number in his usual - as far as I am aware - voice and to me it just diddn't work. I seem to remember that he also committed a Sleepy John Estes song to disc which had the same effect on me. That doesn't mean that he shouldn't have performed them but for me it explains why a "cod" accent is sometimes preferable.
I know from a converastion with Bob that evening that we both shared a love and respect for American vernacular music and if we wish to sing it and enjoy doing so what the heck.

Someone above mentions the Barking Bard, one who does not sing with a cod or "mockney" accent. Personally I cannot listen to him especially when I have heard him do a Woody Guthrie number. But still he is a liberty to do so.

Regarding critics professional or otherwise, the opinions are only that of that one person.

Hoot