The Advance fares, especially with a Senior Railcard, are so much better than a day on a First Class pass that is is well worth seeking them.
The Advance fares are not necessarily released on the same day that tickets are, and they do not apply to all trains on a given day.
Are you looking for them properly? Nigel,found them for October 2. I was conducting a search at the same time and found, for September 29 and 30, a First Class Advance fare of £30.70 on the direct train (no changes) from Wakefield to Glasgow. For Perth to Middleborough, the First class advance fare of £28.50 showed up on the 3-change train for one day, a 2-change train the other.
A 4-day Britrail Pass for 2 seniors traveling together costs $918 on the website. That means the per-journey/day costs or two is $229.50. This is more than double the cost of the Advance fares.
I have some familiarity with your dilemma. Two months ago we traveled around the UK by train. I bought 5 of our 8 journeys ahead and got great Advance fares. I signed up for fare alerts on the East coast trains website and bought the fares as soon as I could. For the other 3 journeys, I decided to purchase a pass because (1) for one journey, Bodmin Parkway to Paddington, my early research led me to believe that no advance fares would appear for the particular journey we wanted, and (2) for 2 journeys with multiple legs I did not want to have to commit us to a particular set of trains that far in advance. As it turned out we decided we should have reservations as we found the trains quite crowded in May, so we had to commit anyway. As it turned out I did predict correctly that no Advance fares would appear on that Bodmin train we wanted! but we paid dearly to ride that train with our pass. So it was a mistake to Purchas the pass buy my travel companions had such a good time on the trip which I planned that they forgave me.
So. . . . Sign up fare alerts and wait for the Advance fares. If you miss them entirely (not likely), you can always purchase the pass later.