what to do with donor car?

[snapper]

AshG is correct, i just add one point to consider, the DVLA ask on their forms how you disposed of the cars remains so you need a receipt for the scrap and a photo of it being taken away.
It's not very joined up i know but DVLA has not fully thought out the issues so it's up to us to prove what we have done and how we did it.

Keep a big folder of all receipts and photos and any correspondence about the build, it will make life so much easier when you come to register.