First,
King Arthur arrived, asking for advice. Yes, that King Arthur. Quite a few people offered their suggestions, Preston among them.
*stares for just a bare moment*
*rips off absolutely perfect RCMP salute before going down to one knee*
Good day, your Majesty. My name is William Preston, a sergeant of the North-West Mounted Police and a loyal subject of the British crown. Under my oath as a member of that force, I believe I'm required to offer you whatever assistance I can.
As I haven't ever been a married man myself, I doubt that my advice on the matter of your lady wife and Sir Launcelot should carry much weight. I can speak only as a man who's seen a great deal of what the law can do, both for the bad and for the good. To take matters at face value and obey merely the letter of your law would cause unnecessary heartache and bloodshed, and possibly deprive your kingdom of a very great ally in the years ahead. If I were in your shoes- though I wouldn't ever presume to such a thing- I would look over the condition of your current kingdom. Is there a place on your far borders that's prone to unrest, or known for lawlessness? Do you have allies or prospective allies on the Continent that you would like to draw more closely to Britain? Is there some great quest or crusade that would do your country honour to participate in?
From my knowledge of history, I wouldn't be surprised if all three of these were the case. If this is so, then I would suggest the following to you.
Begin by offering your farthest-distant Continental ally the presence of one of your great knights at their court. Say that he will speak with your voice and represent your kingdom in all things, save the most important of treaties, which will of course require your personal signature. Suggest to them that the position will be a long-lasting one- say, five years at a time- and that they may call upon this knight to assist them in military matters, just as they might do with England's soldiery in general. Then offer them Sir Launcelot- he is, after all, your dearest friend. It wouldn't be a lie to say that he speaks with your voice, and you could not possibly be construed as dishonouring your ally by sending them a worthless malcontent as your ambassador. The position would be a public honour for Sir Launcelot, but it would also effectively be a punishment of temporary exile. I do not think he could possibly turn it down, either, which eases your position a bit. I realize he is the commander of your army, but it doesn't do your army any good to know that its commander is consorting with another man's wife. I think you'll find your fighting men breathe a bit easier once this is done.
Once you've done this, you must move swiftly to head off both Sir Agravaine and Sir Modred. Call them both in immediately, as well as Sir Gareth. Tell them that you wish to establish a proper legacy of law and order in your kingdom, since other legacies appear to be in short supply- of course, you don't need to say that last. Tell them that you are dissatisfied with the current state of law in general. The majority of it is unwritten, relying on custom and memory- that's no way to run a kingdom! Tell them that you wish to establish a clear, just, honourable and fair code of law, based on both duty and right. This law is to be written down, preserved, and distributed to every title-holder in the kingdom. . . but it must begin with being written out and made consistent. Tell Sir Modred that you want his help because you would have mercy tempered with justice; tell Sir Gareth that you would also have justice tempered with mercy. Then tell Sir Agravaine that you want his help because the law, once sorted out, must be properly enforced. Lay out the principles you want most clearly enshrined in your law, all the rights and duties of the King to his subjects and vice versa, and give them the task of creating this law between the three of them. Give them as much authority as you feel they can handle to incorporate both existing law as it is practiced and the ideal law you would like for people to have, but tell them that you, as King, must have final say over it all. Make sure that no crime, not treason, not witchcraft, not any crime at all, has a single absolute penalty- remember the story of Queen Esther! The law of the Medes and Persians couldn't be undone, but only changed, and that almost led to disaster. You must retain final authority over all sentencing and ability to change the laws.
When the code of the Law is finally written, inform Sir Agravaine that you are going to call all the great lords of your kingdom to your court and require them to swear acceptance and upholding of it, and that you are going to want him and one of his brothers to enforce the keeping of that promise. Ask him to choose the brother he thinks best suited to the task, and set the others to putting out the flame of lawlessness on your frontiers. If any of them seem disinclined to cooperate, offer them a place in a great and honourable quest instead, but do so privately. You do not want to give the impression of exile- only of an honourable alternative. They haven't done anything wrong, after all. Appoint Sir Modred as your counselor in matters of the new, codified law, but choose another- preferably from outside the family- as counselor as well. Justice weighs all things in the balance, after all. Sir Modred should be balanced by someone of less stringent temper, and your sisters' influence should be balanced by loyalty to you personally, rather than loyalty to the family. Possibly the nobility of Wales might be worth asking.
Having done this, inform both your lady Queen and Sir Launcelot of what has happened, and tell them that from this day forward the new laws are going to be enforced on all- great and small alike. Make sure that they know that you have the ability to make Sir Launcelot's exile permanent if it looks as though there will be future trysts. And find someone utterly unrelated to the court to handle the dispatch of messages throughout your kingdom and overseas- someone you can trust to keep messages private.
I hope this helps, your Majesty.
To which Arthur responded.
The short version is that he liked what he heard. A lot.