Taken from DVLA
Special Trailer - Applies to showman’s rigid vehicles over 12,000kg pulling laden trailers over 4,000kg.
Showman’s Haulage Vehicle - The term “Showman’s Haulage Vehicle” means a vehicle which is constructed and used on public roads solely for haulage and not for the purpose of carrying or having superimposed upon it any load except such as is necessary for its propulsion or equipment and which is registered in the name of a person following the business of a travelling showman and used solely by him for the purpose of his business and for no other purpose.
The “Special Vehicle” taxation class encompasses the following vehicles:
Mobile Crane, Mobile Pumps
Digging Machine
Works Truck
Road Roller
Showman’s HGV
Showman’s Haulage, Special Trailer
Vehicles of these descriptions weighing up to 3,500kg pay the PLG rate and tax in the PLG class. Vehicles of these descriptions over 3,500kg pay the basic HGV rate and license in the Special Vehicles Class.
http://www.dvla.gov.uk/~/media/pdf/leaflets/v149.ashx
It’s the DVLA so safe to ‘ok & open’ then click on ‘showman’ on left