"I would suggest that you decide whether you want (mostly) to sing English, Scottish, Irish, or American."
Sorry Richard - crossed lines.
That is my point exactly - sing what you want but do so in your own voice, not anybody else's.
To me, wanting to sing like anybody but yourself is pointless - why try to sound like Shirley Collins say, when Shirley makes a far better job of sounding like herself, so you might as well go and buy her albums.
The joy of singing is interpreting a song for yourself and making that song work for you - making it part of you.
You can only do that in your own voice, otherwise you become little more than one of those sad pub 'tribute' artists.   
I don't believe you can do that using somebody else's voice or accent.
'm not suggesting for a minute that you can't adapt what other singers have to offer to your own singing, but that's a mile away from trying to sound like them
But there you go - that's me (nobody else)!
Jim Carroll