Just to follow on from that....you could have a bog-standard MGB, or Jowett Javelin, or Mark 2 Escort they dont need a logbook or a COD but can enter race meetings as a classic car
I think the COD requirement that Ray mentioned is a bit of a red herring. Isn't it just that a compliant historic or classic car is one that meets one of the Identity Classification / Vehicle Categories under Appendix Six.