What was the purpose of this WWII vehicle?

This beast is a Lagonda flamethrower.

After the evacuation of British Forces from Northern France in 1940, the UK faced an existential threat, invasion and subjugation, similar to the fate of many other European countries. The anxiety of the moment prompted a lot of new -- and sometimes desperate -- thinking. Much of this thinking focused on the deployment of new weapons; a broad range of defensive methods was explored to counter the invasion threat.

Modern-day military flamethrowers had been developed and widely deployed in World War I. Several countries built on these developments in World War II. In particular Britain, through its Petroleum Warfare Department, experimented with several vehicle-borne designs. One of these is illustrated in the photograph. Lagonda, the luxury car manufacturer, cooperated in a project that sought to find a low-cost effective way of protecting lightly armed installations such as Fleet Air-Arm Stations and Merchant Navy ships against dive-bomber attack. The project resulted in various flamethrower designs: this model was based on the chassis of an Associated Equipment Company six-wheeler heavy lorry and became known as a Heavy Cockatrice.

The anti-aircraft flamethrower was not dramatically successful: there is no record of one having brought down an enemy aircraft. However, the Secret Intelligence Service reported that attacking aircraft were forced to fly higher and thus to bomb less accurately.