To take advantage of these prices you will need to prove you are a leisure traveller, on the way to the airport, so you will need to give either your flight numbers or yr holiday booking reference. I don't think you will need to provide this when you arrive at the station but you may be asked to give them on yr journey from brent-cross station to the airport.
Going to the airport on the train is great, you can take a nap, read a book (even at night), play travel scrabble, or just stare 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 grown ups travelling together. But another great benefit is that children go free. All children under five go free, and then one kidover five but under 16 per adult travelling by train to stansted 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 have spare time at the airport you can always read 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.
Or maybe you meant