Even if you don’t do a lot of UK rail travel, take a look at this walkthough I’ve put together for thetrainline.com. It’s amazing how hard they’ve managed to make finding cheap tickets. You have to ignore the search results, click on a textual link, and then select your fare from a page containing - wait for it - up to 150 radio buttons in a huge table - with the tickets differentiated by weird codes like 1ST ADVANCE2 and STD ADVANCE4…
The mind boggles!
Note: the walkthrough may load slowly. It’s best watched in full-screen mode. Thanks to Peter Otto for showing me this tip.