Joe Biden could do this and get away with it. Arguably Obama did, as stated above---there might be better examples. If Charles tried, no one would probably listen, and even if he did, once word got out it would probably actually be the end of the monarchy. Charles is not a dictator, autocrat or anything else. And in fact the US president, though elected, is much further along the spectrum towards autocrat than the British monarch. Many actual autocrats are in fact elected, so this distinction isn't really important anyway.