It was a deliberate decision to not allow hotkeys for items. The game wants you to rely on your own tactics than being able to slam down potions easily mid-combat,. It makes sense from a realistic standpoint, drinking a potion while something is attacking you would be somewhat difficult. It's also the same reason they don't allow hotkeys for spells.

The game has a key for each of the characters inventories, so just keep a potion in the same slot of each characters inventory and it becomes very quick to heal. Sure not as quick as a hotkey but still quick.

Still LoG's system is far better than Dungeon Masters.

Also found this: www.grimrock.net/forum/viewtopic.php?f=13&t=586&start=10

Spell List images attached.

(I didn't make them although Original Symbolic Copyright belongs to the game creators.)

Also Spellcasting tip:
You can enter a spell code outside of battle, just use the X instead of cast to lock it in ready.
Then in battle open the spell selection and just click Cast, sadly this then clears the spell selection...
But I guess it's better than typing the first spell code mid combat.