To take benefit 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 yr 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 provide them on your journey from farnborough-main station to the airport.
Going to heathrow airport on the train is great, you can sleep, read a book (even at night), play travel scrabble, or just gaze out of the window... The other restriction to buying these cheap train tickets from farnborough-main station to the airport is that there must be at least two grown ups travelling together. But another great benefit is that children go free. All children under five go free, and then one child over five but under sixteen per grown up travelling on the train to the airport.
make sure you leave enough time to get to the airport early enough to eat, shop etc, so check the times of trains from the station before you go. If you end up with spare time at heathrow 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 times of farnborough-main station.
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