Lord British can be killed in Ultima 6 using a glass sword. The power of the glass sword is not enough to kill the powerful monarch when he is awake, so wait until he sleeps, equip the glass sword and kill him. He will stand up, but don't worry, he's dead.
See also:
Using a glass sword is the only proven way to kill Lord British in Ultima 6. I have tried other methods mentioned in the Killing Lord British page at The Codex of Ultima Wisdom, but they don't seem to work, at least on the PC version.
There are three ways to steal Lord British's stuff. He's got leather boots, a protection ring, a regeneration ring and a unique Snake Amulet that can be quite handy. Here's how you can do it:
In an older version of Ultima 6, it was possible to put Lord British in a chest and then break him. This is demonstrated in a video (2018) by Tristan Miller, kindly brought to my attention by Wolf Dreamer. The description of the video describes the procedure:
"A demonstration of how to kill Lord British in Ultima VI. Early versions of the game (this one is version 3.2) allow you to push people into containers and carry them around. If you save and restore the game, the game starts treating such people as items rather than NPCs, and they can be destroyed by attacking them."
Things your mother never told you about Ultima 6 by IT-HE Software has a few more things you can do with Lord British, including: