This is what it say's on the DVLA web site

A. Vehicles that have been rebuilt using a mix of new/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/manufacturer (e.g. receipt)

And two other major components from the original vehicle - see list below

Suspension (front & back)
Axles (both)
Transmission
Steering Assembly
Engine

If a second-hand chassis/monocoque bodyshell is used, the vehicle must pass a ESVA/SVA test after which a "Q" prefix registration number will be allocated.