Try contacting DVSA Technical at Bristol or visiting the premises of the nearest old DVSA test station to you, the test lane and enforcement staff are still run from the old stations. Apart from the excise duty position I can’t see much advantage if the vehicle has a design weight of 32000kg, you still have to keep the thing roadworthy. If your business has an O licence the DVSA can still inspect any company vehicle on your premises during a visit.
You mite have a problem doing this ,there is a cut off date or something ,recently they wouldn’t let an x reg 8 wheeler volumetric change to plant from Hgv ,I don’t know why but the firm ended up selling it and buying another .
Muckaway:
Why are they an item of plant but a grab tipper isn’t?
Due to them producing a product rather then collecting or delivering one , fill you body with ballast and cement and use the grab to mix it and you could be the same too