To take advantage 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 give either your flight details or your 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 give them on your travels from the station to gatwick airport.
Going to the airport by train is great, you can take a nap, read (even at night), play travel scrabble, or just gaze out of the window... The other restriction to buying these cheap train tickets from wembley-central station to the 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 kidover five but under 16 per adult travelling on the train to gatwick airport.
make sure you leave plenty of time to get to the airport early enough to eat, shop and so on, so check the times of trains from wembley-central 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 train ticket in advance, you don't need to fret about the opening hours of wembley-central station.
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