A lyric search doesn't recognise the song, so it's almost certainly been written and recorded especially for the ad. That, in turn, means that the advertising agency (Red Brick Road) could have used any singers that they liked and, quite possibly, just booked several session singers (rather than any existing folk group) to perform it.
Thanks to you both, I have searched but drawn a blank. I have a friend who lives in Cornwall and sings for a well known choir but he cannot she’d any light on it but he is probably better placed to dig something up.