To take advantage of the best prices you will need to prove you are a leisure traveller, on the way to gatwick airport, so you will need to provide either yr flight numbers or yr holiday booking reference. I don't think you will need to give this when you arrive at edgware station but you may be asked to give them on your journey from the 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 stare 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 advantage is that children go free. All children under 5 go free, and then one child over five but under sixteen per adult travelling by train to the airport.
be sure to 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 or play travel scrabble again! As you have bought your train ticket in advance, you don't need to fret about the opening times of the station.
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