I think the conference was a early to mid 90's. BSAC approved Nitrox for use towards the end of the 90's. I think my basic 'Nitrox Instructor' ticket is dated sometime around 1998.
One of the issues was that although the BSAC didn't approve Nitrox for use, a lot of the more experienced divers, and senior members of the BSAC where using it.
It was so bad in the early days that some skippers often wouldn't let you on board with Nitrox. So, all the modern procedures for marking cylinders where often not done, cylinders where often disguised as air.