The display of Trade Plates is regulated by the DVLA, but enforced by the Driver & Vehicle Standards Agency through their Enforcement Sanctions Policy which outlines the course of action to be taken when dealing with the majority of all road traffic offences. The document also lists the various offences ranging from seat belts, towing, PSV, HGV, driver licensing and insurance, through to defects and Trade Plates together with the sanctions that apply – eg. fixed penalities.
Trade Plates can be found in Section 3 and the penalties are either a £50.00 fixed penalty or a fine = 5 x duty payable.
Although some confusion may exist, in reality there are 2 scenarios, namely
Unregistered vehicles – Trade Plates are to be affixed in the position designated for the Vehicle Registration Mark or Number Plate and
Registered Vehicles – Trade Plates must be fitted in addition to the Vehicle Registration Mark or Number Plate, which comply with the Display Rules guidance and must be readable from any point measured to a 21.5 metre radius from the number plate centre line
The full Policy can be downloaded from this link>> DVSA_enforcement-sanctions-policy
Relevant excerpt from the Policy