To take advantage of these prices you will need to prove you are a leisure traveller, en route to the airport, so you will need to provide either yr flight details 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 give them on your journey from the station to stansted airport.
Going to stansted airport by train is great, you can take a nap, read (even at night), play a game, or just stare out of the window... The other caveat to buying these inexpensive train tickets from brent-cross station to stansted airport is that there must be at least two adults travelling together. But another great advantage is that kids go free. All children under five go free, and then one child over five but under sixteen per grown up travelling by 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 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 yr train ticket in advance, you don't need to worry about the opening times of the station.
Or maybe you meant