"in the (unlikely) event of total independence for the Celtic countries"

Don't you believe it! (I just came in from a funeral trip to Ireland, and it's late at night, and I haven't even read the thread yet, and someone else may have said that already. But it jumped out at me.

Mind I disapprove of the term "Celtic countries" - Scotland and Ireland and even Wales are by no means exclusively "Celtic", even if you extend that from its proper sense of what language people speak to what language their ancestors spoke. Half the trouble in Ireland has stemmed from people thinking along those lines. But that's drifting the thread.

I'd vote for Yellow Submarine myself. Everyone knows it, it's easy to join in, and the words are highly appropriate and inclusive.