Killing Lord British has become an accomplishment in itself over the years. Richard Garriott had something against the players trying to assassinate his character Lord British. At first Garriott made this task extremely difficult, but program oversights still made it possible. Later, the whole thing was seen more humorous. Therefore, here now are the known ways to kill Lord British in the games:
Lord British is invulnerable to normal weapons in Ultima III -- however, he isn't to ship cannons! The Player can lure Lord British towards the moat, after making him aggressive, and kill him with the cannons of the ship parked there.
Bumbling around for too long in the game (meaning several years) will result in Lord British dying from starvation during his imprisonment, which also means a game over.
Finishing the game without having the Sandalwood Box in your inventory causes you to be trapped with Lord British with no means of escape, which is another means of a game over. Presumably, you and Lord British both eventually starve to death while trapped in the alternate dimension if this happens.
Every day at noon, Lord British stays in the doorway to the throne room. Reading the plaque above the door will cause it to fall, beheading the king. This was inspired by an incident at the Origin building; a metal bar, where the magnet on the door attached to it, fell on Garriott's head, warranting a visit to the hospital.
It also is possible to use The Black Sword against Lord British, which results in a humorous scene before the deed is done.
Using the Armageddon spell, it is possible to annihilate every single active object in the game. High-level spells cannot be cast in Britannia; however, it is possible to cast Armageddon in another world, and then return to Britannia by dying. It is impossible to reach to the main floor of the castle to check if the spell has actually killed Lord British, but it stands to reason that he is dead because the spell destroys every object in every world, including ones that are normally indestructible.
The Avatar can use his bread maker together with rat poison to make poisoned bread. This can be taken to Britannia, and exchanged with Lord British's meal in the kitchen. Lord British will come in, eat the bread, and die from the rat poison.
The whole thing also accidentally happened in Ultima Online (see top-of-article image). A character called "Rainz" used a stolen firewall-scroll while Lord British gave an audience. Richard Garriott, who had forgotten to activate his "Lord British-invincibility" must have looked quite shocked, when his alter ego dropped dead on the spot.
In Ultima VII, If you use the Exult cheats to enter God Mode.. Use the Armageddon spell in LB's presence. Then attack him. He will attack you without mercy even if you use charm on him. Using sleep on him will just cause him to continue to attack you after he wakes up. The only way to make him friendly towards you is to kill him by luring him into the courtyard and having the plaque fall on his head. Resurrect him afterwards and he will be friendly towards you again.. Of course, all he will talk to you about is your irresponsible usage of the Armageddon spell.
If you use the cheat mode, use the pickpocket cheat on Lord British and you will find not only his scroll, but a lightning bolt. You can arm it, but not use it as an attack method. Batlin will have a fireball on his person, in his left hand, and a tuft of sparkles in his right. (he can only attack you with his hands if you take these from him). In his pack he has what looks to be the amulet of Lord British itself. If you drop it, it will be too heavy to pick up again. But you must use the Exult Cheats to accomplish these feats.