Whilst Charles is technically "Commander in Chief" (I think I've got the role right), will he be the one pressing the scary big red button with "Annihilate the Planet" on it? I suspect not.Apart from the government and the armed forces no.
If you go down the US route, then the President does have that capability I believe, but here we have a Head Of State that is more geared up towards being a relatively apolitical constant.
I'm by no stretch of the imagination a fan of the monarchy, but electing a power crazy lunatic every 4 years does not seem like a better system to me - that's part of the reason why I don't believe that our current system of government is really working as there are too many power-hungry career politicians in the House of Commons these days.