As talked about in another thread, the TOCs▸ expect you to book in advance if you want to travel at an affordable price now.
I think that's what's called a gross - and pretty inaccurate - generalisation - I don't think anyone could seriously call many
FGW▸ walk-up off-peak fares unaffordable, especially not if you are in an area where a railcard can be used as well - eg Network Card area, Devon & Cornwall, Cotswold Line. I consider my ^5.30 Moreton-in-Marsh to Oxford
CDR▸ on a Cotswold Railcard excellent value, it's less than petrol would cost alone, never mind putting another 60 miles on the clock on the car if I did drive - and less than i was paying last year, thanks to a fare cut in January, one of quite a number in the Thames Valley and Cotswolds, following up those in the South West mentioned previously.
That's not to say that some peak fares are value for money, nor can it be denied that there are lots of anomalies about, but that's been the case ever since they stopped charging by distance travelled.