I can't speak for Britain, but in Canada, it's not just that the people would get upset if the monarch decided to use the land differently. They are legally una

I can't speak for Britain, but in Canada, it's not just that the people would get upset if the monarch decided to use the land differently. They are legally unable to. The courts are not subservient to the monarch, and the monarch is not above the law. Note that there is already legal precedent in Canada for the courts telling the monarch (through their representative, the Governor General) that their power is merely symbolic in nature. Not just a mere cultural norm, but enshrined through legal precedent. Charles can't do whatever he likes with the land without breaking the law.