I'd guess British Rail used to sell Super Advance fares for that route, but Virgin Trains would have surely phased them out quite early on and replaced them with their Virgin Value fares?
Perhaps someone with knowledge of fares on the West Coast route at that time could say...?
GNER used to use the term Super Advance for several years, along with Apex etc, and as East Coast currently restrict the use of the lounge, I'd guess it's likely that GNER did, so if you remember the lounge being restricted and the ticket type Super Advance, is it possible you were thinking of GNER?