It’s all to do with the maximum weight the vehicle could be (potential, not actual), that’s what determines what speeds are permissable. That’s why one Berlingo, with seats and windows, can do car speed limits, and another one, in a van style, and built to carry a higher weight, (maybe) can’t. 2 tonnes max laden weight is the cut off.
Cut and pasted from here…
…What is a “car derived van”?
Under the Road Traffic Regulation Act 1984, a “car derived van” is defined as:- “A goods vehicle which is constructed or adapted as a derivative of a passenger vehicle and which has a maximum laden weight not exceeding 2 tonnes.”
The important word in this definition is “and” as there are goods vehicles that look as if they are based on a passenger vehicle, but when the manufacturer puts a gross laden weight on the goods vehicle, which is the design weight of the vehicle plus the maximum load that it is designed to carry, and this exceeds 2 tonnes, that vehicle is no longer a car derived van. The van becomes an ordinary goods vehicle under 7.5 tonnes gross weight, and is therefore subject to the speed limits as shown in the Highway Code.
As a “rule of thumb” any van larger than a Vauxhall Astravan will have a gross weight in excess of 2 tonnes and is therefore subject to the reduced “class of vehicle” speed limits. Ford Transit, Mercedes Sprinter, Mercedes Vito, Peugeot Expert and Ford Connect, for example, are restricted vans.
It is immaterial that a goods vehicle may be unladen at the time it is detected exceeding the speed limit. The construction of the vehicle that enables it to be used up to the 7.5 tonnes maximum weight is the relevant criteria.