To add to the suggestion by Dizzy, midway through the practice rain ride just stop. When you stop, stand there for an hour in the cold rain just like you would if you had to change a flat tire on the rear wheel or maybe wait for a SAG wagon if you had a bigger breakdown. Fixing a flat tire when you are cold, wet, tired and everything is slippery is much more complex than doing the same task on your patio table on a nice warm day. Many riders can push through even when cold and wet because of the body heat they generate . . for awhile. However If all of a sudden a bike problem brings the plan to a halt, now you have an added problem. But even this test ride my be deceiving. Ones tolerance to the cold/wet is dependent on your physical condition at the time. Hunger, dehydration, and exhaustion all play a role.
Like Mark47, I have spent considerable time in the backcountry, some of it in the mountains where it rained almost every day. I have yet to meet any individual who is immune to hypothermia.