from which year / date of 1st reg warning lights are mandatory?

Pretty sure it's registered after 1980 for hazards, not required if q plated as they are taken as 1974 or similar. As usual if fitted they must work no matter what age.
All cars need them at IVA though. I guess you are talking about an older car rather than new.

not required for: A vehicle first used before 1st April 1986.
Though I think you have to have them for IVA

[Edited on 6/11/16 by gremlin1234]