So far as the Purchase Tax avoidance was concerned, and based on my understanding from the various bodies on Lea-Francis on the one hand and my memory of what friends did on the other a van was a commercial vehicle and not subject to PT. However an Estate Car/Shooting Brake/Utility was not a van, the significant differences being that the van has no side windows or doors aft of the driver's door.
After a certain time from new, one could cut windows in the side without paying PT - and I think if you did so earlier there was a sliding scale of PT liability. I remember people with Ford 100E vans cutting windows in them, but in the case of Lea-Francis the wooden bodied vans simply had to have panes of glass fitted where plywood panels had been.
This Bristol, as illustrated, is not a van so would have had to pay Purchase Tax