I still don't know why the British would imprison Hess instead of negotiating with him ... Hess wanted to negotiate

Hess was clearly deranged*, and had no credibility as a negotiator. The terms apparently offered were laughable, and later, after America entered the war, the only acceptable terms were unconditional surrender, so there was nothing to negotiate. On the other hand, his arrest was a propaganda coup for Britain at a time when they were thin on the ground. His post- war detention was the result of power play between Russia and the west.

* Even more deranged than the rest of the Nazis.