One could argue that by not having staffed ticket offices or ticket machines at every station, and no guards / conductors on trains (or lazy ones), they are giving their services away. They are not necessarily losing money by failing to have adequate ticketing facilities.

Barriers must be left open if there are no staff around who are available to open them (which can be done remotely at some stations, though, AFAIK, only from elsewhere in the same station).

Honourably, if there was really no opportunity to buy the ticket that you wanted during the journey, you can always buy a ticket online later and not collect it.

I usually buy rail tickets online and >80% of the time they are not checked although they could have been. However, recently I decided not to buy in advance if I am not legally obliged to (generally this means starting from a non-PF station or wanting a complex ticket) - and if the train operating company does not want to collect my fare then I won't pay.