There is now a Government consultation on "Vehicles of Historic Interest".
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/554763/historic-vehicles-consultation.pdf
This suggests that the 8 point rule might be used to determine if a car was "substantially changed" or not.
I think this is the way forward and would ask people to reply to the consultation questions if they can.
As personally, I want my kit car to become historic when it reaches 40 (currently a spritely 36 years old).
The key will be that it is 8 point compliant, even if it is no longer a "standard" classic Triumph Spitfire.
Cheers,
Paul.
PS
More details on this Rods 'n' Sods thread:
http://www.rodsnsods.co.uk/forum/topic/new-consultation-historic-vehicles-roadworthiness-testing-467625