Can you elaborate? At one point or another I'm sure each TOC's website has had a sale selling its own cut-price tickets, along with the usual non-sale ones.
e.g. East Coast/Virgin "flat fare", LM "half price", TPE "student 50% off"?
Full requirements are given in the
Ticketing and Settlement Agreement, but the basic summary is that an impartial point of sale isn't allowed to favour one TOC over another, so if London Midland's site, for example, showed their Advance fares for London - Birmingham, and only showed the Anytime Return fares for Virgin and Chiltern, that would be a breach of the impartiality rules. (A rather extreme example, admittedly, and one that shouldn't happen assuming Advances are available at the time).
Offering a discount for buying your ticket from a TOC website is perfectly legitimate, and a commercial decision for the operator concerned, it doesn't stop the site from being impartial providing you can still buy tickets for journeys on other TOCs.
Similarly, if you went to the main ticket office at Victoria (which is run by Southern) and were looking to buy a ticket in advance to travel to Gatwick, they should ask further questions to determine what the most suitable ticket is, be that one for First Capital Connect, Gatwick Express or Southern.
If, on the other hand, you went to the Gatwick Express ticket office at Victoria (assuming it's still open), they're only obliged to sell you a Gatwick Express ticket, as they're not the lead retailer. (And yes, I'm well aware that Gatwick Express is a brand of Southern, but for this particular purpose they're separate).
HTH,
Barry