thetrainline does show you what the cheapest ticket is in a box but it forces you to select a service before it will sell you that ticket which probably leads many people to believe that you have to use the trains selected regardless of the ticket type. Then adds on those fees so you've no longer got the cheapest ticket!
I deal with that misunderstanding on a regular basis.
Does the new RSH interface require you to select a specific service when booking a non-Advance ticket? I haven't been able to get the interface to show up for me, so can't test this myself.