At the moment the oldest livery is the GNER blue being carried by a few Class 91 locos.
When these have been repainted, I think the oldest will be the Virgin Trains livery as on the Pendolinos and Voyagers, which is from 2001 (correct me if I'm wrong on this one though).
I thought the oldest livery was on some of the the Greater Anglia 321s in the old First Great Eastern Livery. I'm willing to put the FCC Class 313s as the oldest interior, despite the new seats, the wall panels and flooring still dates from the late 70s.