To take benefit of the best prices you will need to prove you are a leisure traveller, en route to heathrow airport, so you will need to provide either your flight numbers or your 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 your travels from the station to the 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 restriction to buying these cheap train tickets from milton-keynes-central 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 5 go free, and then one child over 5 but under 16 per adult travelling by train to heathrow airport.
be sure to leave enough time to get to heathrow airport early enough to eat, shop and so on, 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 fret about the opening hours of milton-keynes-central station.
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