I think since the very early days, the government has set "parliamentary" fares for basic 3rd class travel (1d a mile etc), for all-stations services.
For a long time railway companies also paid a duty on higher fares for faster trains and higher classes, until the duty was abolished in 1929.
The "fares book" was just a single sheet of paper with a grid of mileage, classes and fares, which tapered slightly at the higher end.
There really weren't categories of fares like Advances and Off Peak like there are today, nor time restrictions, just "the fare".
On the other hand there were more special excursions and one-off services, eg to sports events, at special fares.
Freight was relatively more important than it is today and formed the bulk of traffic/profits on many routes.
The railway companies had to accept traffic however unprofitable (common carrier obligation) right into the BR period.
Basic safety legislation was strictly enforced, and I think employment was protected to a degree (although pay was very low).
The railways also agreed common rates on certain neighbouring routes (eg Manchester-London via LNW, Mid, GC, GN), and had many joint services where it suited them.
This is bit like the airline code-sharing practices today, sharing costs/revenue on a pooled set of routes.
So the Big 4 (or their predecessors) were not necessarily as competitive as today's TOCs.
The beggar-thy-neighbour era of unbridled competition ended roughly in the 1860s when the big railways generally called truces with their neighbours.
I think the railway companies were more driven by costs, most of which were under their direct control (track, rolling stock, operation, maintenance etc).
Today, most of these costs are externalised from the TOCs by contracts with NR, ROSCOs and manufacturers, leaving just the train operational costs to control.
We are inclined to concentrate on the TOC accounts, but something like 75% of their costs lie elsewhere and are mostly fixed for a franchise duration.
The NR costs, in particular, are not subject to competition (though ORR tries to regulate them).