There is some very definite legislation associated with the rebuilding and altering of vehicles, and the whole story can be found on the DVLA website https://www.gov.uk/vehicle-approval
But in summary...
Conditions for retaining the original registration.
For you to keep the original registration of a vehicle you MUST retain the ORIGINAL shell or replace it with a BRAND NEW shell to the same specifications ( Heritage are the ONLY supplier of new shells for the mini ) and at least two further components from the original vehicle.
- Suspension (front & back)
- Axles (both)
- Steering Assembly
So technically if you have a rolling shell, you have enough of the original vehicle.
If those conditions are met, when the car is rebuilt it only need to undergo an MOT
Using a 2nd hand shell
If you are intending to use a second hand shell to rebuild a vehicle, and you dont have enough of the shell donor ( or the registration document for the vehicle ) to use that registration, then the vehicle MUST undergo an ESVA/SVA, (Extended) Single Vehicle Approval, from which it will be give a "Q" plate registration, before it can be taxed and used on the road.
Failure to disclose this is paramount to falsifying the identity of a vehicle, or ringing to use its more popular term, and is a Criminal Offence
This is an area of contention, as they are neither new shells to the original specification, and also not necessarily used shells.. so this could come under the Kit car part... where you may be able to get an age related plate and or the original ( check with the DVLA on this one )