Actually I can suggest a reason you can’t get a reasonable priced AP. i have experience the same issues. The problem is actually TPE not having opening reservations.
i tend to book the AP from York - Scotland then but the York - Destination separately. So the reason you often can’t get an AP is because TPE don’t realise then on existing stock. (Because it is too small)
would you not rather wait 6 months and have you core route fixed first?
Wonder if TPE will offer cheap advances between Edinburgh and Newcastle and points further South?