Are online ticket prices lower than those bought at the station at the time of travel, advance priced tickets excepted ? I am thinking in terms of the "same day" type of ticket at £8.90 versus a ticket bought to travel on the next available train. The route is London Bridge to Gatwick on a weekday mid morning.
Online tickets for off offpeak and anytime tickets are the same on the day as online - indeed cheaper if the online seller charges a fee.
But that £8.90 fare is not a Thameslink ticket - it's a Southern one. They operate a slow stopping service from London Bridge to Gatwick which takes rather longer than the Thameslink (FCC) one - that costs £10