Geoff to echo what Kevemm says and give you my own personal experience of the V5C descriptions of body panel kit cars situation - completely out of the blue one day in back in 2007 I received a note (see attached) from my local DVLA office (when they still had local offices) asking to inspect my car.
https://www.dropbox.com/s/lbiz1h71ed...acted.jpg?dl=0
When I phoned them up to ask if it was about the F355 kit I had put on my MR2 they refused to say. At this point I got somewhat annoyed and asked what would happen if I didn’t turn up for the appointment, to which they replied that I would not be able to re-tax the vehicle until the inspection had been done, effectively putting my car off the road in under a year.
On the day of the inspection they phoned me up and asked me to bring all my documentation relating to the kit I had added to the car. When I got there they looked around the car and I explained how the kit was added to the existing monocoque body. They were satisfied that I had still retained the identity of the original car (as I had not altered the monocoque) and they proceeded to change the description of the car on the V5 so I left a happy chappy.
It is also my understanding, although I have never heard of it happening, that an MOT inspector is quite within his rights to refuse to MOT test a car if the description of the car does not match its appearance.
I have since built a 250 GTO body panel kit for which I have also had the V5C description changed. There is certainly a lot more awareness of this requirement amongst buyers of completed kit cars to the extent that not having had the V5C description changed can effect the value of the car.