To take benefit of the best prices you will need to prove you are a leisure traveller, on the way to the airport, so you will need to provide either your flight details or yr holiday booking reference. I don't think you will need to provide this when you arrive at the station but you may be asked to provide them on your travels from the station to the airport.
Going to gatwick airport on the train is great, you can take a nap, read a book (even at night), play travel scrabble, or just gaze out of the window... The other restriction to buying these inexpensive tickets from the station to gatwick airport is that there must be at least two adults travelling together. But another great benefit is that children go free. All children under 5 go free, and then one kidover 5 but under 16 per adult travelling by train to the airport.
make sure you 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 the airport you can always read or play a game again! As you have bought your ticket in advance, you don't need to worry about the opening hours of the station.
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