There is a benefit to an app in situations with congested networks, Waterloo at rush hour used to be awful for data.TBH it's just as easy to set up an "app like" link from your browser. The main advantage of apps is the ability to work offline, there is nothing to gain by doing that with RTT.
I have been made aware of one quite prominent system internally in the industry (with a greater circulation than just TRUST) which uses +/-. TRUST isn't entirely consistent in its use of L/E, either. There are several pages on it where it won't say anything other than the number for a delay, and an E for early. I believe there's one area where it uses negative values but I can't remember what it is off the top of my head.Its just entirely inconvenient when every other source I use sticks to E/L.
Even the at times bizarre TRUST sticks with the conventional answer to this question.
Either way, it is really not a big deal. Railways across the world use ± and RTT is not going to be contained to just Great Britain in time. In the grand scheme of things it is so minor it's almost amusing how much of a riot it's causing.