Chapter 2/Stage 2 noise regulations came in to force on 1st January 1986.
The Trident did not meet these new regulations and BA's last Trident operations were on 31st December 1985. They finished with simultaneous approaches to 27L and 27R at Heathrow - one was service flight and one was a charter. Certain people on both flights could then spend hours arguing that THEY were on the last Trident service!