To take advantage of the best 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 milton-keynes-central station to heathrow airport.
Going to heathrow airport by train is great, you can sleep, read a book (even at night), play a game, or just gaze out of the window... The other restriction to buying these inexpensive tickets from milton-keynes-central station to the airport is that there must be at least two adults travelling together. But another great benefit is that children go free. All children under five go free, and then one kidover five but under sixteen per adult travelling on the train to heathrow 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 yr train ticket in advance, you don't need to worry about the opening hours of the station.
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