When I was in grade three I noticed that the children around me sang with a subtle British accent. This was strange because none of us had British accents otherwise, but somehow, when we sang in music class, our vowels would warp until we became distinctly...British.
I was probably sensitized to this because I was listening to Pink Floyd's "The Wall" at the time, which contains that "Hey teacher" children's chorus on "Another Brick in the Wall." THOSE kids sounded British too, but they actually WERE British, and I'd somehow picked up on the fact that when WE sang, WE sounded like THEM.
Around this time my father started insisting that us Southern Ontario people didn't HAVE accents; he believed -- and still believes -- that we have become the lowest common denominator of English speech, a mix of all possible accents together until we don't HAVE one anymore. He justifies this by claiming that when people sing they always sound like US, and therefore singing must somehow REMOVE accents, resulting (I suppose) in some sort of pure, undiluted English.
As a child I could never put my objections to this theory into words, but now I suspect that my father was analyzing a certain subset of music, probably bands subsequent to the British Invasion. And it's true, when you listen to bands from England during that period -- Led Zeppelin, say -- many of them DON'T sound British.
But something I realized while listening to my classmates sing was that singing is not something "pure" that releases us from our speech patterns...at the very least it is just another type of affectation. If Julie Andrews could sing in a Cockney accent during "My Fair Lady" even though she wasn't actually British, it's obvious that a British singer could sound North American in exactly the same way...which stands to reason, considering many of those British Invasion bands were emulating American blues and skiffle musicians, and many of them craved acceptance on the Billboard charts.
If you listen to a British band that is NOT trying to be self-consciously American, however, you hear a very thick accent. It might not sound EXACTLY regional -- because, as I said, singing involves affectation and imitation just as much as any other activity does -- but a band like "The Pipettes" certainly doesn't sound like they come from Southern Ontario, that's for sure.
None of this explains why us little Canadian kids sounded British when we sang, however, but I've noticed that it isn't just children who do this...a subset of Canadian artists sound self-consciously British when they sing. Maybe, as kids, we were imitating British new wave bands? Maybe it just seemed like the best way to sing? Or maybe there's some germ of British inflection that comes out when we let our guard down?