To take benefit of the best prices you will need to prove you are not a business traveller, en route 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 give this when you arrive at the station but you may be asked to give them on yr journey from brentford station to stansted airport.
Going to the airport on the 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 inexpensive 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 child over five but under sixteen per adult travelling by train to the airport.
be sure to 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 have time to kill at stansted airport you can always read a book or play travel scrabble again! As you have bought yr train ticket in advance, you don't need to fret about the opening times of the station.
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