According to my wife, who is neither Royalist nor Republican, but studies the history of England’s Kings and Queens, King George V laid down the rules in 1917.
The titles of Prince and Princess are restricted to the children of the reigning sovereign, e.g. Prince Charles, the children of the sovereign’s sons, e.g. Prince Harry, and the eldest son of the sovereign’s son, e.g. Prince George..
She made it patently clear in the first five seconds, that no one was being paid for the interview, whether she had her fingers crossed when she said it, I didn’t notice.
It was said twice that it was neither the Queen, nor the Duke of Edinburgh, but let’s be fair, they can keep the speculation going for ever just by saying that they’ll never divulge the name.