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 provide this when you arrive at the 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 a book (even at night), play a game, or just stare out of the window... The other caveat to buying these cheap train tickets from the 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 5 but under 16 per grown up travelling on the train to gatwick airport.
make sure you leave plenty of time to get to gatwick airport early enough to eat, shop etc, so check the times of trains from edgware station before you go. If you end up with spare time at the airport you can always read or play travel scrabble again! As you have bought your ticket in advance, you don't need to fret about the opening hours of edgware station.
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