To take advantage of the best prices you will need to prove you are not a business traveller, on the way 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 wembley-central station but you may be asked to give them on your journey from the station to the airport.
Going to gatwick airport on the train is great, you can sleep, read a book (even at night), play travel scrabble, or just gaze out of the window... The other restriction 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 advantage is that kids go free. All children under five go free, and then one child over five but under 16 per grown up travelling by train to gatwick airport.
make sure you leave enough time to get to the airport early enough to eat, shop etc, so check the times of trains from wembley-central station before you go. If you end up with time to kill at gatwick airport you can always read a book or play a game again! As you have bought yr train ticket in advance, you don't need to worry about the opening times of the station.
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