Lol Don't worry with that Simonrazer, please stick with the Nodes or I'll get lost
Let's make this only with nodes please.
You see the UI can be, and in most cases are, just planes and other objects, just like everything else
For example you add 3 tinny Jets in the bottom, above the entire scene, and every time the game Jet gets hit you make one tinny Jet disappear. Once all the 3 jets disappear you insert the text "Game over" (again a object).
I made exactly that on my blender asteroids and works perfect:
Everything are objects. Here it is the .blend file: https://app.box.com/s/2z90vncmelfrpzbz513c
For the health bar you can add 2 planes, one black for background, with the full health (1) and a red one, on top of the black, that will scale from zero to 1 As soon it reaches the full bar one tinny Jet disappears.
I'll then edit them and will make them pretty