I would suspect that if it fitted the weight/braking requirements and towed correctly as a steered trailer it may seem to be a trailer. However, I suspect the Police will apply common sense and say that 'if it looks like a car and moves like a car then it must be a car, albeit a non running one.
Maybe you could argue that it is only a car once it has been through the IVA and been registered.
The next point would be that if it is recently made, and I can't remember the year, the 'trailer' will need to have a plate that states the year of manufacture, number of axles, unladen and gross weights, etc. and the Police can make you take it to a weigh bridge to confirm the figures on the plate.
Of course, if it is above the maximum unbraked weight you car is allowed to tow, 750kg maximum but could be lower, then it will also need to have overrun brakes with auto reversing. That would be quite a difficult thing to fit on a kit car.
For moving mine about I am designing a skeletal trailer to carry it. Basically an axle and drawbar that inserts under the chassis.
If I can keep my trike and trailer combo below the 650kg unbrake limit for my car then I can use an unbraked trailer.
I could dispense with the chassis and just bolt the suspension units to the underside of the trike chassis and then bolt on a drawbar but then the area becomes a little greyer!
