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 numbers 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 provide them on your travels from the station to stansted airport.
Going to the airport on the train is great, you can sleep, read a book (even at night), play travel scrabble, or just stare out of the window... The other restriction to buying these inexpensive train tickets from the station to the airport is that there must be at least two grown ups 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 16 per adult travelling by train to stansted airport.
be sure to leave enough time to get to the airport early enough to eat, shop etc, so check the times of trains from forest-hill 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 yr train ticket in advance, you don't need to worry about the opening times of forest-hill station.
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