To take advantage 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 give this when you arrive at the station but you may be asked to provide them on your journey from bracknell station to heathrow airport.
Going to the airport on the train is great, you can take a nap, read (even at night), play travel scrabble, or just stare out of the window... The other caveat to buying these inexpensive train tickets from the station to heathrow airport is that there must be at least two adults travelling together. But another great advantage is that children go free. All children under five go free, and then one child over five but under sixteen per adult travelling by train to the airport.
be sure to 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 end up with time to kill at heathrow airport you can always read or play a game again! As you have bought yr train ticket in advance, you don't need to fret about the opening hours of the station.
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