Britain has a constitutional monarchy, where the monarch has a formal role in the formation of governments but an obligation to remain neutral and no practical political power.
Charles has been criticised in the past for his views on topics such as the environment and architecture, but said he recognises being heir to the throne and head of state are two different roles. But in an interview with the BBC, the Prince of Wales was quick to play down any concerns that he would continue to do so as king, saying "I'm not that stupid".
Asked what advice she would like to give him for his 70th birthday she adds: "My Father once said to me, 'As you get older, you've got to do one thing and that is pace yourself". "I do realize that it is a separate exercise being sovereign, so of course I understand entirely how that should operate".
The documentary captures the prince in both public and private, including images of him feeding vegetable scraps to his chickens and collecting their eggs at his Highgrove home.
Asked if he believed he had established a template for the future, Charles replied: "I doubt it, I mean, you know, my eldest son William may want to do to it differently, slightly".
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The documentary features interviews with his sons Prince William, who said he would like to see his father spend more time with his grandkids, and Prince Harry, who praised his father for walking Meghan down the aisle when her own father couldn't make the wedding. "Because when he's there, he's brilliant, but we need him there as much as possible".
He's better known for speaking the Queen's English but the Prince of Wales on Wednesday turned to pidgin as he visited Nigeria's commercial capital. "I do fervently pray that you'll all do your utmost to work together in the coming year to make real, substantial progress", he said.
The BBC was given exclusive access to Charles, who will turn 70 on November 14. "Their Royal Highnesses have appreciated all of the support they have received from people around the world since their wedding in May and are delighted to be able to share this happy news with the public".