This is an old chestnut covered ad nauseaum on other forums. It would be worth pinning this otherwise the question will be asked time and again.....
The general consensus has been that since T5.1 Kombis have been registered as N1 (Panel Van / Light Goods) and not M1 and so are restricted to 50/60/70. M1 covers cars so Vels, Calis and possibly shuttles so they can do 60/70/70.
By and large taxation is done by vehicle class so N1 is LGV and M1 is passenger car and therefore CO2 emissions.
This page sets out speed limits quite clearly;
Speed limits - GOV.UK, or so you might think.
It refers to ‘car-derived van’ or ‘dual-purpose vehicle’ which if you follow the links you will find;
"Dual purpose vehicles
A dual purpose vehicle is a vehicle constructed or adapted for the carriage both of passengers and of goods and designed to weigh no more than 2,040 kg when unladen, and is either:
- constructed or adapted so that the driving power of the engine is, or can be selected to be, transmitted to all wheels of the vehicle
or
- permanently fitted with a rigid roof, at least one row of transverse passenger seats to the rear of the driver’s seat and will have side and rear windows - there must also be a minimum ratio between the size of passenger and stowage areas"
From which you will see that a Kombi and indeed a lot of conversions may fall into the category of Dual Purpose.
Further if you have a conversion and get your V5 update so the body type becomes "Motorhome" or "Motor Caravan" (note you cannot change the classification from N1 to M1 just the body type) then the speed limit becomes 60/70/70 but your tax remains LGV (and you may get cheaper insurance) so it is worth doing.
This page is a good source with clarification from VOSA;
Motorhome Speed Limits
If you get caught by a camera then it appears that they are just using the N1 classification and it seems that others don't know the whole story either. So as long as they are showing you evidence that you were exceeding the van speed but not the car then tell them and if they persist go to court.