I was worried that a handlebar bag would hang too low and block the light on my bike. It looked to me that the Carradice Super C handlebar bag didn't hang down too far below the mounting point, so I took the plunge. Now I have it mounted, and indeed the bottom of the bag is above the top of the light, and the light isn't blocked.
This is a size 565L Nomad Mk2 and I have one spacer between the headset and the accessory bar. There are another two spacers between the accessory bar and the stem.