Yes, I'm aware of the 1/76:1/72 issue with the older Airfix kits. Apparently somebody actually took them to court over it quite some years ago.
I ran across a reference to building a Commer tractor for the Queen Mary in a 1978 Airfix book on RAF Vehicles which I found on Amazon and is winging its way to me. The whole book looks very interesting.
As an aside, apparently while the Austin K2 in the Emergency Set is considered pretty good, but the fire tender that accompanies it bears little resemblance to any WWII vehicle.