To take benefit 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 provide either your flight numbers or yr holiday booking reference. I don't think you will need to give this when you arrive at the station but you may be asked to provide them on your 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 a game, or just gaze out of the window... The other caveat to buying these cheap tickets from london-kings-cross station to stansted 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 five go free, and then one child over 5 but under 16 per grown up travelling on the train to the 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 london-kings-cross station before you go. If you end up with time to kill at stansted 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 hours of the station.
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