I can't agree that singing a song is necessarily a performance. I am pretty sure that most people sing for their own enjoyment of a song because they like that particular lyric and or melody.
I remember many years back I used to sing for my own enjoyment "Pub with no beer" and to sing it in anything other than an attempted "strine" accent made it sound ridiculous. Likewise I have a liking for the songs of the American south. To sing about catfish, possum, grits and gravy etc. in an english accent sounds equally so.

Regarding Glasgow friends being most appreciative of and englishman singing "Rothesay-O", I suspect that they were being polite. I had a similar experience when I was asked to sing by a couple from North Carolina. I sang an american song and when I finished the husband turned to his wife and said "see when he sings he don't have an accent". Very polite those southerners.

Hoot