To take benefit of the best prices you will need to prove you are not a business traveller, en route to heathrow airport, so you will need to provide either yr flight numbers or your holiday booking reference. I don't think you will need to provide this when you arrive at st-margarets- station but you may be asked to provide them on your travels from the station to heathrow airport.
Going to heathrow 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 restriction to buying these cheap train tickets from the 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 kidover 5 but under sixteen per adult travelling by train to the airport.
make sure you leave enough 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 end up with spare time at the airport you can always read a book or play travel scrabble again! As you have bought yr ticket in advance, you don't need to worry about the opening hours of st-margarets- station.
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