I can actually believe that the journey time hasn't increased since the 1950s. However, possibly the clock at the end of the film is there to indicate the differing journey times?
Anyway, interesting video. thanks for posting.
You can see when they go through the first big tunnel (the merstham tunnel http://en.wikipedia.org/wiki/Merstham_tunnels) they enter at the same time and exit at the same time so the old one must be (maybe??) sped up.
It would have been interesting to see what Gatwick airport was like then but it doesn't really show it, other than there being a big space on the right hand side that isn't there any more (2:11).
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