But unless it can be proved that the shell has had it's ID altered (as opposed to having an ID given to it) then it's not illegal.

How would we go about proving the shells original ID if we suspect it is a pre 83 car which has had it's ID removed?

If it was a new pre 83 shell that then had the ID of a written off S1 put onto it at a legitemate body shop doing a legitemate repair, then it's not illegal for example.