Some reasons the British might not negotiated with Rudolph Hess:
*Hess flew into England unannounced.
*He was not in some neutral country, say Spain or Portugal to conduct such business.
*When he was captured he gave mixed signals about why he was there, and whom he wanted to see.
*While he was a Deputy Fuhrer, it was known he increasingly was becoming out of favor with A.H. Could this have been a ruse?
*If he was what he said he was, and they sent him back, he likely would have been killed by Hitler.
*If he was not there for what he said he was, they would send him back to continue planning & participation in the war against the allies.
*What he seemed to be offering was peace with Britain only, primarily so that Germany could send resources to the Eastern Front against the USSR. So the war would, in any case, continue.
*By imprisoning him, the allies might get valuable information from one so high in the German Wehrmacht.