That is, is there a way to determine this without doing an actual search for each route on multiple days to see if the price changes?
I am traveling to London and considering several day/overnight trips from the city, and would like to know which rail trips I should try to book ahead of time to save money, and which trips I could book at any time without fear of the cost changing.
I am aware that travel in the AM peak into London and the in the PM peak out of London is often more expensive. However, for some routes, the price seems to vary quite a bit beyond this peak-period distinction, based on when you purchase the ticket.
For example, the cheapest Edinburgh-London train I can find leaving tomorrow is £55, and many trips cost much more. But if I look in mid-June, the same train (at the same time, on the same day of the week) starts at £30 -- obviously better to book early.
In contrast, a single trip from Bletchley to London costs either £19.50 (on trains arriving into London before 10:00) or £15.50 (all other). That's it -- you could book an AM peak trip for late June and it would cost £19.50, or you could book it for tomorrow and it would still cost £19.50, so in this case booking early would have no advantages (cost-wise) for me.
(There is perhaps a more concise way of stating the question -- the UK National Rail ticketing system has me a bit confused so I apologize if I haven't understood something.)