You will be looking at this design then. A 1920 'Admiralty Pattern' upgraded in 1924 with balloon tyres on solid wheel rims and because of the chassis series used, curvaceous front wings.

- 1924 Mk1 Armoured Car.jpg (61.46 KiB) Viewed 675 times
These were used in more theatres of War than the earlier design.
The general dimensions are pretty close to the car on display in Bovington Museum.
To be more accurate for making a model of this car, the Matchbox #Y-36 Rolls-Royce Phantom I would be a better starting point, but the Y-15 is ideal for the 1914-1920 Armoured Car, it all depends on your own modelling skills at to how good or accurate you want them.
GHOSTHUNTER.