To take advantage of the best prices you will need to prove you are a leisure traveller, en route to gatwick airport, so you will need to give either yr flight details or yr holiday booking reference. I don't think you will need to give this when you arrive at the station but you may be asked to provide them on yr travels from peckham-rye station to gatwick airport.
Going to the airport by train is great, you can take a nap, read (even at night), play a game, or just gaze out of the window... The other caveat to buying these cheap train tickets from the station to gatwick airport is that there must be at least two grown ups travelling together. But another great benefit is that children go free. All children under five go free, and then one child over 5 but under sixteen per adult travelling on the train to the airport.
be sure to leave enough time to get to the airport early enough to eat, shop and so on, so check the times of trains from the station before you go. If you have spare time at gatwick airport you can always read or play travel scrabble again! As you have bought your ticket in advance, you don't need to fret about the opening hours of the station.
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