I assume it's some kind of tax angle with a pick up counting as a "commercial vehicle"and thus a much lower "benefit in kind" compared to the £80k luxury vehicles they increasingly resemble. The intention of the rule is not to unfairly penalise a builder legitimately needing a transit, or a farmer his Landy.
That's the size of it, the pick up has to have a load capacity of 1000Kg, and is registered as a commercial, the B.I.K is taxed as a van, however they are subject to the same speed limits as a van, so 50 Mph on NSL single carriageway, 60 Mph, NSL Dual Carriageway, 70 Mph on a motorway, it seems most of the muppets that buy these things have no clue about this, as I would regularly be passed by them on the A19, where the police speed camera van likes to sit on the bridges above the A19, when you spot them, too late, there's a fine on it's way! iirc I think one of the Ford Ranger ones, doesn't have a 1000Kg load capacity, & is registered as a car, but I may have misread that somewhere