Lots of questions posted for Gatwick trains but did not see these.
1. An advance online round trip ticket is available off-peak for £13. Is this a significant deduct for that available for a same day purchase at the station? If I can purchase the same ticket at the station for say £14, I would probably do that. Similarly, would an advance ticket purchase to Portsmouth be a sigificant saving over that purchased at the station.
2. Can you purchase an advance ticket that can be used on any day? Example - If I discover that Westminister Abbey has an event scheduled (therefore closed to visitors) I might want to change the day of travel into London, switching days with our Portsmouth plan mentioned above. I am not seeing a way to do that on the Southern Rail site.