There is a lot of misunderstanding on the internet etc. in respect of Dual Purpose Vehcles and speed limits.
The definition of a Dual Purpose Vehicle is in The Road Vehicles (Construction and Use) Regulations 1986, (nothing to do with the registration details in that definition - many are N1, Light Goods, Panel Van etc), all that is required is that the vehicle meeets that definition.
Dual Purpose Vehicles are in the same group of vehicles for speed limit purposes as cars, motorcycles, & car-derived-vans in Sch 6, The Road Traffic Regulation Act 1984.