I would call that fairly regular
Sorry to be thick but I am still super confused.
So if a Gatwick Express branded train, leaving from the Gatwick Express platforms at Victoria, has Brighton at its destination then it is a Gatwick Express train (in terms of fares) for anybody going to Gatwick but a Southern service (in terms of fares) for anybody going to Brighton? And for anybody joining at Gatwick it is a Southern service from Gatwick to Brighton?
Does the Next Fastest Train screen at Victoria show these services at the next fastest Brighton service?
Then in the reverse, if a train branded Gatwick Express from Brighton to London a Southern service in terms of fares until it gets to Gatwick where it becomes the Gatwick Express fares wise?
Presumably the written timetable then had some sort of note that explains this change in status half way through the journey?
Then the exception to the above is where a Gatwick Express branded train is used on service that is in the timetable as a Southern service and that is just a Southern service (and you have to rely on the boards on the platform to make that clear)?