It sounds like the new test will follow a similar format to the SVA where one-off builds are concerned. One point I have noticed whilst researching on the web is that there may be a new form of MOT introduced,the test will take longer and part of its criteria is to compare vehicles to standard (or point of single vehicle approval) and if the car has deviated from approved standard identify if DVLA are aware. This could give rise to a re-test.
This could potentially put an end to people passing their car for example with a pinto engine and then sticking a Hayabusa in the following week (might take a bit longer than a week
)
Cheers
foxcub