The Seabrook was a commercially acquired lorry with armoured bodywork and armament added to convert it to a fighting vehicle.

The Seabrook lorry was chosen to mount the armoured body, which was built and fitted by Portholme Aerodrome Limited at Huntingdon.

They were intended to provide heavy fire support to the machine gun armed Lanchester and Rolls-Royce armoured cars.

Although the first armoured lorry had given little trouble, and proved the concept of a gun-armed fire support vehicle, the large crew and heavy armament overloaded the Seabrook chassis and in service the springs, the wheels with their solid rubber tyres, and the back axle often failed.

Because of its mechanical difficulties and poor off-road performance, the decision was later made to restrict the Seabrooks to formed roads.