This is the reply I got to a similar question I posed to the DVSA:-
Approvals,
Customer Service Centre,
DVSA,
Ellipse,
Padley Road,
Swansea,
SA1 8AN.
Good afternoon Richard,
Thank you for your enquiry regarding this matter which has now been passed to me, as the Technical Officer advising on M1, N1 & O1 Individual Vehicle Approval (IVA) matters.
If you were to make a new trailer, then yes it would require Individual Vehicle Approval (IVA) prior to it being used on the road.
If you were to convert a car or trailer to make a new trailer, then yes it would also require an IVA prior to it being used on the road.
If you were to take an existing trailer and simply mount a new body to it, then no it would not require an IVA prior to it being used on the road.
This is covered in paragraph 5 (Scope of the Regulations) of Part 1 of the Road Vehicles (Approval) Regulations 2009.
I have attached further information regarding the IVA scheme, which I hope will be useful and can be found by following the links below:
VEHICLE TYPE APPROVAL - https://www.gov.uk/vehicle-approval/when-you-need-vehicle-approval
IVA - https://www.gov.uk/vehicle-approval/individual-vehicle-approval
(Includes links to the Model Report list on the VOSA site, list of Technical Services, the IVA Guide, Test Stations and POTFs)
IVA MANUALS - https://www.gov.uk/vehicle-approval/individual-vehicle-approval-manuals
IVA FORMS - https://www.gov.uk/apply-vehicle-approval
Glad to know that because that is exactly what I am doing.
All the IVA consists of is meeting various specs. Nobody actually checks if the thing tows properly or the brakes actually provide enough stopping power. Tick box mentality!