But surely the whole point is that the anthem is "The National Anthem". You do not need to buy-into every one of its phrases in order to sing (or just stand and salute) it.
Singing 'God save the Queen' is a statement of loyalty to the queen as the figurehead of the country. Just as a Scout's promise to "Do my duty to God and to the Queen" is not a promise to go round to Buck House, and clean up after the corgis, but a pledge of loyalty to the country which the Queen represents.

To say that it should be dropped because 20% are republicans is pointless. In Wales, the fervour with which the National Anthem (Hen Wlad Fy Nhadau) is sung (paticularly at rugby matches) is amazing. But for many it is just memorised. I would be surprised if as many as 20% could tell you what the words actually mean. But the anthem has a single meaning, Welsh patriotism. The words (as the Spanish would tell you) hardly come into it.