I just checked the clock times, and it's not possible to judge.

1953: no departure time, arrival is 13:05.

1983: departure at 14:00, arrival at 14:03. Fast trip! Or someone cheated setting up clock / departure shots.

2013: departure: 08:32, arrival 09:50.

The Brighton-London trip looks to be about 1:10 based on this site: http://traintimes.org.uk/?page=/service/timetable/lateroutbo...

Trip times here are 58m to 1h 20m. https://www.thetrainline.com/train-times/london-to-brighton