To take benefit of these prices you will need to prove you are not a business traveller, on the way to stansted airport, so you will need to provide either your flight numbers or yr holiday booking reference. I don't think you will need to provide this when you arrive at london-kings-cross station but you may be asked to give them on your journey from the station to the airport.
Going to the airport by train is great, you can take a nap, read (even at night), play a game, or just gaze out of the window... The other restriction to buying these inexpensive train tickets from the station to stansted airport is that there must be at least two adults travelling together. But another great advantage is that children go free. All children under five go free, and then one kidover 5 but under sixteen per adult travelling by train to stansted airport.
make sure you leave enough 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 end up with spare time at the airport you can always read a book or play travel scrabble again! As you have bought your ticket in advance, you don't need to worry about the opening hours of the station.
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