To take advantage of the best prices you will need to prove you are not a business traveller, en route to the airport, so you will need to give either your flight numbers or your holiday booking reference. I don't think you will need to provide this when you arrive at farnborough-main station but you may be asked to provide them on your journey from farnborough-main station to the airport.
Going to the airport on the train is great, you can sleep, read a book (even at night), play travel scrabble, or just stare out of the window... The other restriction to buying these cheap tickets from farnborough-main station to heathrow airport is that there must be at least two grown ups travelling together. But another great advantage is that children go free. All children under five go free, and then one child over 5 but under 16 per grown up travelling by train to heathrow 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 farnborough-main station before you go. If you end up with spare time at heathrow airport you can always read a book or play a game again! As you have bought your train ticket in advance, you don't need to worry about the opening hours of the station.
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