I'd forgotten about this "enhanced standard class" you mention.
Silver Standard I think it was called, and I think the idea was that you had to have what we would now call an "Anytime" ticket to travel in it. They would of been as rare as hen's teeth on Cross Country routes of the time I'd have thought, as on many flows (e.g. Reading to Birmingham) reduced price tickets were valid on ALL trains.
Actually, Silver Standard was ahead of its time in anticipating a market for premium standard class business travel- at the time my aunt used it regularly as she was on an NHS relocation package from Liverpool to Sussex and would book the Silver Standard carriage to come back to Liverpool on a Friday evening. The NHS would pay for standard class rail travel and nothing more, but as long as she produced receipts for standard tickets they didn't query whether she could have paid less and used Savers- at the time the Liverpool service was every 90 minutes from Euston so I doubt there were anything like the peak restrictions that exist now.