I had this same issue (just returned to UO and forgot about not patching), and there is a fix, at least on Windows 7. If you right click on your "Ultima Online Classic" folder (or whatever folder holds your client) and hit properties, there's a previous versions tab. Copy the entire folder of a version that's before the patch. Swap the two folders out and you should be good! I renamed the up to date one and hung onto it, just in case something went wrong. Hope that helps!
It's also a good idea to delete any shortcuts that UO created, as they point to autopatch, instead making them point to client.exe.
I know there used to be a way to get UOA to bypass patching, but I see it's already pointing to client.exe, and runs the patcher anyway. Anyone know of a way around that?