We seem to have come full circle and arrived back where we started.
There are in essence three broad types of vehicle recognised in UK law, these being "motor cars", "goods vehicles", and "dual purpose vehicles". These categories are defined by the Road Vehicles (Construction & Use) Regulations 1986 (as amended). [Within some of these categories there are sub-categories, such as "heavy motor car" etc. Anything not in one of these categories can only be driven on the road by virtue of Special Types regulations.]
If you drive a Landrover Defender, then irrespective of what it is used for, who owns it, how it is taxed, you are NOT driving a light goods vehicle, you are driving a Dual Purpose Vehicle. It may be taxed in a VED category called "N1 Light Goods" but it is not a light goods vehicle.
Any vehicle which is a Dual Purpose Vehicle as defined in C&U(86) must be MOT tested as Class 4 and is subject to normal car-type speed limits. That is the law, to do anything else is illegal.
It is true that a Class 7 MOT tests the same things but to a more rigourous standard tha Class 4, but if you lose income because your vehicle has failed a incorrectly-administered Class 7 test when it should have passed a Class 4 test, I imagine you could now sue the test station. Similarly if you have presented the vehicle for test and the test station has carried out and charged you for a Class 7 test when it should have been Class 4, they have overcharged you (or mis-sold you the test).
You can argue the absurdity of the situation, but you cannot argue the facts of the situation.
That was, IIRC, the intention at the time. The message given out to the trade was that we would be changing to the Euro style M1/N1 passenger / goods classification and DPV would be completely removed making it 'easier' to determine goods vehicle as based clearly on GVW/DGW.
Unfortunately your selective quoting of my post is exactly the sort of thing which is propogating the myths on this subject, since it suggests that there was an intention to remove the DPV vehicle type definition from the C&U(86) regs. This has NEVER been the stated case, and DVSA and/or the DfT cannot do this - only an Act of Parliament or Statutory Instrument can alter the C&U Regs. What DVSA proposed was a change to the MoT Test Regulations, which would have removed the provision that states that DPVs are tested as a seperate defined category. It would only have affected MOTs, it would not have affected speed limits, tachographs, drivers' hours, weight limits, type approval, or any of the other areas where DPVs are/were/may be treated differently and specifically.