The way the rail fare system works is that with longer journeys, a certain number of tickets for each journey are sold as "advance" tickets, these are up to 75% off the walk up on the day price but are restricted to a specific train at a certain time. They go on sale 12 weeks before the date of travel, once the 75% off advances are sold, the next slightly more expensive advances become available until all of them are sold out and so-on until no advances at all are left. On some services, it's possible to purchase a relatively cheap advance the day before travel. It is possible to switch one advance for another for a small fee and any difference in price between the two. It's also possible to split-ticket journeys so long as the train stops at all of the stations where the journey is split. Of course, airlines generally have a bit more flexibility in how they price and sell tickets because they are not subject to franchising.
The main advantage the railway has business-wise over regional airlines is that most train operators will always be there. Look at the East Coast Main Line for example. It was originally privatised as GNER, then once they weren't financially viable any more it became National Express. NE subsequently handed it back to the government who a few years later re-privatised it as Virgin East Coast. It's now been re-nationalised again. Most of the staff are TUPE'd over from one organisation to the next. In the UK, the only train companies that can go properly bust are freight operators and the open access passenger operators which are not subject to franchising and whom make use of spare track capacity.
Unless there is a nationalised regional airline, or one that largely operates PSO routes underpinned by government cash, then I think the regional airline industry will have a tough battle against rail. Things will only get more difficult as additional rail infrastructure is developed.