I give up! I've already posted one Steve marrioyy vid ( 03 Oct 18 03.25) and he is clearly not singing in any sort of UK accent; indeed, he throws in one of those " you know what I'm talkin' about" blues clich comments. AND, he's pure US even when he's speaking!
Dave, why should Steve and Robert be different than nearly every other UK blues'rock artist who sang in an America accent?
Well, they weren't!

I don't know why you mentioned Kate Rusby because, surely, no British singer who specialises in UK/Irish folk music uses an American accent.

And quoting The Proclaimers is surely shooting yourselve in the foot because EVERYBODY mentions their Scottish singing voices but nobody mentions the Scottish accents of ANY Scottish pop/rock artist.
Go back and listen to Lulu from the 60s belting out "Shout" in her great Scottish accent...not!