An interesting element to consider regarding situations like this is: where does power lie? In a milieu that at least nods toward "realism" power lies in political structures. The king is powerful because the barons follow him because he's powerful (with some Divine Right thrown in). There is no need for the king to be a high level badass because the ability to win battles is single combat is not the defining factor of civil authority.
I agree. But I would implement this more along "realism" lines . . .
King: You shall go to the dungeon of despair and bring me the McGuffin. You may keep any other treasure you find, and you shall have my thanks.
PC: What's in it for me?
King: My thanks can take many forms. We shall discuss what is appropriate upon your safe return with the McGuffin.
PC: That's not good enough. We need money, and we need to know how much before we decide if we're going to do this for you.
King: This quest is for the defense of the realm. The decision is mine, and it has been made. You will go, and you will have my thanks, if you do not try my patience further. Be gone with you!
PC: Wait.
Guards: Time to go. Come with us.
PC: Hey, you can't order me around. I'm the Wizard who killed the blue dragon of Ohm!
Guards: Move now. Last warning.
And if they fight, well, maybe they can kill the king and his guards, but they are in big, big trouble with rest of the realm. You could try and beat up the British Prime Minister, and maybe if you're Rambo and friends, you can succeed at taking out his bodyguards. But you'll be persona non grata with pretty much everyone in the civilized world who knows you did it, especially if you did it for a punk reason like he dissed you . . . most likely you'll be hunted down and killed in short order.
"Unnecessary violence in the apprehension of Jake and Elwood Blues, has been approved."