Soldato
I seem to remember from when I last did a speed awareness course there was some discussion about speed limits for vans, car derived vans and 'grey area' vehicles such as vehicles which were based on the same model but sold in van and MPV forms and pickups. From memory, in these cases which limit applied was determined by the vehicle classification on the V5 rather than simply weight and it was entirely possible for almost identical vehicles to be subject to different limits simply because they were registered differently. I'm not going to say with any certainty which speeds apply to which classifications for fear of giving out misleading information but it's worth you finding out how your truck is classified and starting your research from there.