To take advantage of the best prices you will need to prove you are a leisure traveller, en route to the airport, so you will need to provide either yr flight numbers 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 journey from wembley-central station to the airport.
Going to the airport by train is great, you can take a nap, read a book (even at night), play a game, or just gaze out of the window... The other caveat to buying these cheap tickets from wembley-central 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 5 go free, and then one child over 5 but under 16 per grown up travelling on the train to the 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 the station before you go. If you have time to kill at gatwick airport you can always read a book or play travel scrabble again! As you have bought yr train ticket in advance, you don't need to worry about the opening hours of wembley-central station.
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