This sounds great as an urban legend and all, but why not just remember "1-12"?
I don't need a song to remember there's some significance to each number 12 and under.
Off the top of my head:
1. God
2. Two Divine Natures of Christ.
3. Trinity
4. Four cardinal virtues
5. 5 sacred wounds, 5 books of the Pentateuch (Books of Moses)
6. 6 days of Creation
7. The Sabbath day God rested
8. The Eight Beatitudes
9. Nine choirs of angels
10. The Ten Commandments
11. The 11 faithful Apostles
12. Twelve tribes of Israel, Twelve Fruits of the Holy Ghost
No song necessary.
In fact, I'll go further: the song does NOTHING to nudge you or help you remember the significance of any of these numbers. All it does is remind you of the EXISTENCE of these numbers -- which, again, you don't need. If the words helped you IN ANY WAY to remember the significance of a given number, maybe you'd have a point. But none of the "twelve days of christmas" items are anything close to what it represents.
Oh, and why would Protestant England -- even during the Penal Times -- be against the Pentateuch, the 6 days of creation, or the Apostles? Couldn't those have been more explicit in the song? Why all the code words and secrecy? Anglicans are creationists too, right?