Interesting, Are you meaning that the station wagon versions with the seats in the back are treated the same as the hard tops without rear windows and seats?
(Direct from the VCA)
As stated previously, there is no taxation class such as a commercial vehicle.
All vehicle taxing is governed by the Vehicle Excise and Registration Act 1994 (VERA). Classification for taxation purposes depends on the construction of a vehicle and its use on the public road.
The Light Goods Vehicles taxation class applies to goods vehicles (mainly light vans) registered on or after 1st March 2001, which are 3500kg or less and have a type approval category N1.
An M1 passenger carrying vehicle 3500kg or less such as a Petrol car or Diesel car, could not be changed to LGV.
Type approval is the process which ensures that vehicles, their systems and components, meet the appropriate environmental and safety standards for the appropriate vehicle category. Vehicle manufacturers apply to the Vehicle Certification Agency (VCA) for the type approval category relevant to the design style of their vehicle.
The existence of a Type Approval Certificate shows that the vehicle conforms to standards of design, construction and environmental protection.
For further information relating to type approval categories, you will need to direct your enquiry to the following address:
The Vehicle Certification Agency
1 The Eastgate Office Centre
Eastgate Road
Bristol
BS5 6XX