?? Not sure what you're getting at here? The chorus is "From Ushant to Scilly is thirty-five leagues". It doesn't say which way you're going.

It would not be unreasonable, surely, to heave to in order to get an accurate sounding. After which you start beating up the Channel. The song is a song, with a loosely accurate progression of landmarks that a mariner of the time would recognise.

That's it. Where's the problem?