Minecraft wiki entry for Ultra_hardcore_survival, associated text: First of all, as stated before, there is no natural health regeneration. So, remember to avoid even the most pathetic ways of getting damaged, such as being spat on by a llama or touching cactus.
You'll have to be really quick with getting resources, because no regeneration makes taking damage much more dangerous.
Find ingredients to make a golden apple, a potion of Healing, suspicious stew with regeneration, or a potion of Regeneration as quick as possible, as these are some of the few ways to regenerate health in UHC.
Although hard and labor-intensive to acquire, a beacon can make your life significantly easier, along with reducing the amounts of potions and apples you may consume.
Remember hunger is the most crucial part of survival. As with a normal survival, get food quickly. Hunger is a pitiful excuse for taking damage.
Keep a water bucket on your hotbar at all times. Falling and fire damage can be reduced by using a water bucket.