This is what the DVLA says about this topic.
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.
If a second-hand chassis or monocoque bodyshell is used, the vehicle must pass a an enhanced single vehicle approval (ESVA) or single vehicle approval (SVA) test after which a "Q" prefix registration number will be allocated.
All well and good but if you bought a decent original vehicle to start with, what's stopping you other than your conscience, from replacing major components over time as happens normally when a vehicle ages?
I mean a lot of our Landies have all had engine transplants, replacement galvanised chassis', different gearboxes, axles, parabolics etc, and i don't believe the owners tell the DVLA anything? (other than engine number changes i suppose) .
Chris