Military Vehicle Registration
In the inter-war period I believe military vehicles were customarily registered with the London Count Council and so carried the appropriate plates, and I recall seeing a convoy of army lorries in the late 1930s the plates of which were all lettered in the HML series. I particularly noted this as those are my initials!
Of course this became impractical early on in the war, when the system was replaced by using RAF or whatever was applicable, followed by a very long string of digits. However it was early days when Geriaviator's picture was taken, when the new method had yet to be applied (across the Channel, anyway).