A cheat file was developed by Clockwork & Stix to enable cheating in Ultima VIII. You can download it from various locations, including:
Another cheat can be activated by following certain instructions (which are on Prescient Dragon's Complete Guide to Ultima VIII - Pagan).
The Cutting Room Floor has an Ultima 8 page which describes how to enable the Quotes and Endgame menu options, as well as how to enable the debug mode.