I didn't either until a couple months ago and its actually quite convenient. There's an app called
Trainline where you:
1. Enter your two stations
2. Select return/single/etc...
3. Tap the train you want to go on
4. Choose the cheapest ticket available. (off peak tickets will only show for off peak trains I'm pretty sure...)
It gives you two QR codes arranged nicely in a tab for 'out' and 'return'. Just scan the QR code on the scanner at the rail station if it has it. (if not, just show it to a rail worker, they'll let you through) The scanner is this thing:
View attachment 128714
I was quite afraid of using at first but its now my preferred way of of getting a ticket. No need to faff with the annoying, dirty ticket machines. The app should have this logo:
View attachment 128715
I use it to check train timings as well cos idk how else to do it lol. The platform predictions have been accurate 100% of the time for me, I'm not sure about others tho...