I do not think there is much comparison with the airlines - every point of entry is manned and secure and tickets (for which there is compulsory reservation and therefore valid on one specific flight, usually for a specific seat) rigorously checked for validity before one gets anywhere near a plane. Over-riding ones ticketed destination is just not a practicable possibility. There aren't the equivalent of railcard discounts etc, and adult passengers purchasing child tickets are easily found by the compulsory ID production. The airlines have almost no reason to prosecute for ticketing offences. The fare system may be as byzantine as the railway's, but this does not matter as they are entirely computerised and compulsory reserved so the passengers have little possibility of using tickets for anything than was straightforwardly intended.
The problem is that the railways are mass transit and carrying vast quantities of passengers daily, and the amount of front line staff time available per passenger is very, very limited. Passengers have very diverse expectations and it is simply not possible, or economical, to cater for every possibility. You only have to read some of the threads on this forum to see how many different people somehow expect the railway cater for their particular financial circumstances, their individual journey, their particular desire of comfort, their particular need for attention etc etc. Add to that the propensity of some to exploit loopholes, commit fraud, travel without paying, antisocial behaviour etc means a plethora of rules and threats of prosecution. Of course, the people who would never dream of doing any of these things then feel aggrieved that they are affected by these rules and threats. Those who are between the two camps will have differing views too.
I don't think Ticket Office staff have ever generally had enormous knowledge of the range of ticketing products, or of the train services on offer, other than in their local area. In the past, the range of ticket products/fares was much less than it is now, but the explosion in this has been as the railway has tried to cater more and more differing customer requirements in order to boost revenue, which has been successful in growing the market, both reducing subsidy and permitting greater frequency of service. But this has been at the expense of a confusing ticket system to the outsider.
I doubt it is economically sensible to train Ticket Clerks to the extent of the aficionados in this forum and elsewhere, especially as the industry would probably rather move towards the airline model of ticket / reservation sales outside of local / suburban lines.