To take advantage of these prices you will need to prove you are not a business traveller, on the way to the airport, so you will need to give either yr flight numbers or your 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 yr travels from the station to the airport.
Going to the airport by train is great, you can sleep, read (even at night), play a game, or just gaze out of the window... The other caveat to buying these cheap train tickets from brent-cross station to the 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 grown up travelling on the train to the airport.
make sure you leave enough time to get to stansted airport early enough to eat, shop and so on, so check the times of trains from brent-cross station before you go. If you end up with time to kill at stansted airport you can always read a book or play travel scrabble again! As you have bought your train ticket in advance, you don't need to fret about the opening times of brent-cross station.
Or maybe you meant