"We would never comment on private correspondence"

This quote by Prince Charles is most likely describing the reality that he and his household are often the subject of public analysis and speculation. By stating that they would never comment on personal correspondence, he is highlighting the importance of appreciating the personal privacy of the Royal Family. He is likewise likely trying to prevent individuals from speculating about the private lives of the Royal Family, as it is not proper to do so. This quote is a tip that the Royal Family should be treated with regard which their personal lives should stay personal. It is a reminder that the general public ought to not be trying to pry into the private lives of the Royal Family, as it is not their location to do so.