Network Rail have spent billions upgrading and improving a system that was run into the ground during WW2 and was then simply patched up for decades by BR, who, as everyone old enough will remember, ran such a fantastic, clean, punctual service with helpful, friendly staff (*NOT*)
If company X sells cheaper tickets for a slower, stopping service then clearly you cannot use that ticket to travel along the same line on a faster service which charges higher prices. You, the passenger have to choice. With BR you had no choice.