To take advantage of the best prices you will need to prove you are a leisure 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 give this when you arrive at finsbury-park station but you may be asked to provide them on your journey from the station to heathrow airport.
Going to the airport by train is great, you can sleep, read (even at night), play a game, or just stare out of the window... The other caveat 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 five go free, and then one kidover five but under 16 per grown up travelling by train to the airport.
make sure you leave plenty of 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 have spare time at heathrow airport you can always read a book or play travel scrabble again! As you have bought your train ticket in advance, you don't need to fret about the opening hours of finsbury-park station.
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