Vehicles that have been rebuilt using a mix of new or used parts
In order to retain the original registration mark:

cars and car-derived vans must use:
The original unmodified chassis or unaltered bodyshell (i.e. body and chassis as one unit - monocoque); or a new chassis or monocoque bodyshell of the same specification as the original supported by evidence from the dealer or manufacturer (e.g. receipt).

And two other major components from the original vehicle - ie suspension (front & back); steering assembly; axles (both); transmission or engine.