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 your flight details or yr 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 provide them on yr journey from the station to the airport.
Going to the airport by train is great, you can take a nap, read a book (even at night), play travel scrabble, 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 child over 5 but under 16 per grown up travelling on the train to gatwick airport.
be sure to leave plenty of time to get to the airport early enough to eat, shop etc, so check the times of trains from the station before you go. If you end up with 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 mill-hill-broadway station.
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