Scroll Lock key is the keybind to completely hide the UI.

You can also set a macro if you want, that changes between one hudlayout to the next, and hide everything that you'd wish to hide while not in combat - even if it's just one or two hotbars.

would simply be /hudlayout [insert number of hudlayout here]

This has the added benefit of leaving the chatbox up, as well as being able to leave your map, inventory, menu etc up.