To take benefit 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 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 provide them on yr journey from northampton station to gatwick airport.
Going to the airport on the train is great, you can take a nap, read (even at night), play travel scrabble, or just gaze out of the window... The other caveat to buying these inexpensive train tickets from northampton 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 kids under five go free, and then one child over five but under sixteen per adult travelling by train to the airport.
make sure you leave plenty of 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 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.
Or maybe you meant