Boundary Zone 6 to Southend Stations is £10.50 SDR, £10.10 CDR. This is a good way to get a ticket from the London area to Southend that is valid via both routes.
Boundary Zone 6 to Reading is actually restricted by route, but is not TOC specific and both routes go to the same station so you can buy the cheaper and excess to the higher one to go via two routes, or you can use the more expensive ticket via the shortest route (the guard can either accept it or charge a £0.00 excess).
Boundary Zone 6 to Reading is £10.70 SDR route Ascot, or £16.80 SDR Route Slough, and for the CDR it is £10.60 route Ascot and £9.40 route Slough.
It's more useful for Southend given the fact the routes are completely different and go to different terminals.
I suggest you get a copy of Avantix Traveller. It was leaked and the URL made available via the
PF appeal blog but this became unavailable about the time you joined the forum. ATOC want to keep the fares data available only to people prepared to pay £9+P&P+VAT. I see nothing wrong with copying the software as the fee is less than the admin fee charged to refund a ticket, and that can be waived at the discretion of the ticket office. In effect I conclude we are being charged an admin fee to be issued with the software rather than paying to have the right to use it. People can disagree and make their own choice.