Ooh, I forgot to add the correct fuse in line.
You are right, this set up is absolutely fine for side light illumination but for headlights, you just need to wire in to the fuse box rather than off one of it's smaller feeds. That way, you can connect to a more suitable cable drawing the right current. If you wire to the fuse box, you don't need an in line relay as the feed to take it from will have a relay.
Oh, and the point of the diode is to stop electricity flowing back to the interior lights (or wherever you take the feed) therefore by taking a feed from the interior lights, you won't melt the loom, it will just not have enough power to work the headlights properly.