Yes full fare comes up as an absolute rip off for me (adult and 3 kids from Glasgow to Manchester) if I split my journey it costs 1/3 , but apparently it's frowned upon. A friend of my husband was the one who put us onto it but he mentioned recently you have to be careful because @Manchester Picadilly the guy checking his ticket got really arsey about it more than a few times.
I have no issues with ticket splitting.
If the train operating companies were more honest with their pricing, you would be automatically paying the total cost of the split fare without the need to split tickets. Tickets are becoming more expensive through dishonest pricing in addition to the inflationary increases.
Another tactic is to increase the price in a certain direction. Usually services towards London on Southwestern Railway suffer with this. The same journey starting at the destination station in the reverse direction usually being cheaper.
For example, my local station to Twickenham is £10.55 return. If I started at Twickenham and travelled in the reverse direction to my local station, it is £7.90 - £2.65 cheaper.
I've only had one occasion where a ticket office staff member was a bit arsey, but it isn't my problem. I'm only concerned about getting to where I want to go for the most reasonable price possible.
Apparently ticket office staff are briefed NOT to advise and encourage split ticket fares - even though they are legal.
There are a whole host of websites and apps that now advise. Although some are better than others. Many of the apps lack a 'via' option, which can sometimes be crucial in obtaining the best possible fare.
The guy who works in my local station ticket office recently stated 'well done for doing your homework' when I purchased a split fare from him. It really depends on the member of staff, some are more jobsworth than others. Athough they have no right in being arsey at anyone over a split ticket. Staff that do can be reported.
(This is why I don't fully support the rail strikes. Some staff need booting out for the abysmal customer service they provide, not protected by the union for doing so, nor lavished with a huge pay rise for providing crap customer service - although I have encountered some good staff in my time, so not all are bad.)
It isn't illegal, nor breaking any railway byelaw, as long as you ensure your tickets are valid.