Leyland Dafs CAN be downrated to 7.5 tonnes GVW, mine has been! when fitted with a box body, mine has a gutted electronics repair body, and fitted with a fixed bed, cooking facilities & storage facilities and NO LOAD CARRYING CAPACITY it can be legally registered as a motor caravan. Road tax is £165.00 per year (Private Heavy Goods) the Mot class is large clas 4. I rang VOSA for advice before doing this. The unladen weight given by Whithams must be wrong as my truck still displays a 9 tonne bridging plate. dedeucting the 4 tonne payload gives an unladen weight of 5 tonnes.
Personally I found Withams rather unhelpful, I bought my truck from Dave Crouch, of Crouch Recovery, who not only downrated it, but also Moted registered and taxed it, at extra cost naturally, so all I had to do was pay for it and drive away. Remember if you buy a vehicle from Withams first you have to transport it from their yard, then get it insured, downrated, MOTed, then registered, all a lot of hassle & expense.