The thing is, it's a European Community whole vehicle
type approval system.
Whole vehicle type approval is the process by which vehicles, their systems and components are approved to the appropriate national, European enviromental and safety standards. Without it, vehicles cannot be sold or registered in the UK.
There is a UK specific type approval, to help low volume producers, called National Small Series Type Approval. The down side to this, is that you can only sell these trailers in to the UK.
Of course, you could do an IVA for each and every trailer you built, but that would get tedious, quite quickly, I should imagine.
Bit of info, if anyone's inclined: Trailer Certification