DVLA operates a point scoring system to determine if a vehicle is actually that vehicle or not. Engine change is 1 point, engine and box I think 3, axles are scored and some other bits and pieces.
A new monocoque body shell or even a chassis on its own can lose you the registration number, unless it has been supplied new with paperwork to support and is the same as the original.
So highly modified vehicles will generally end up on a Q plate, and will require a SVA / ESVA test to show fitness for road as they are not type approved.
I think that covers the basics, have a search for DVLA and modified vehicles, I recall the DVLA website has the relevent info.
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