Rose was wrong!
Rose was wrong when she chastised Eddie for getting the name of the flag 'wrong'.
It is a myth that the flag is only known as the Union Jack when flown at sea. This myth has to do with the flagstaff on ships. However, the British flag was known as the Union Jack many decades before the 'invention' of the flagstaff.
On top of that, Rose refers to the Union Jack in at least one other episode.
I will add this info more eloquently to the article later, unless somebody else does it first.