a certain section of the crew of any naval vessel in commission at the time who would be in urgent need of education

Is there anything in that song that would not be done at the command of an officer of some degree? Why would a rating need to know the landmarks ? It was sufficiently unlikely for the story to be told that Cloudesly Shovell had a seaman hung as a mutineer for suggesting that his navigation was wrong when approaching the English Channel. True or not Shovell's navigation was wrong and they hit rocks off the Isles of Scilly with the loss of all lives.

The Ushant to Scilly information only became a matter of significance to the Royal Navy after the introduction of a certain strategy in the 18th Century

Ushant and Scilly are the landmarks (and hazards) on either side of the entrance to the English Channel. Thirty five leagues is not a big target when entering the western approaches not knowing your longitude other than by dead reckoning. That's why the 'fine sandy bottom' is important because helps know were you are - the nature of the bottom was and is marked on the charts.

Admiral Shovell's disaster was 1707. The tricky nature of getting into the English Channel before you could see one of the sides was a matter of significance long before that.