The short answer is: book at eurostar.com as soon as the trains from London St Pancras to Brussels, Calais, Lille or Paris go on sale for your chosen dates, and travel off-peak.
Like airlines, Eurostar uses dynamic pricing: the prevailing fare level depends on demand. For every departure, the train operator decides how many seats it wants to sell at each price level, and when the cheaper tickets sell out, the price rises. Unlike airlines, Eurostar rarely nudges down prices to stimulate demand – while easyJet or Ryanair might reduce the fare for a slow-selling flight a month or two before departure, I have never seen Eurostar do that for trains.
Eurostar's lowest year-round return fare is £58. It is most likely to be available for departures from noon on Monday until the last service on Thursday, and for Saturday services from noon onwards.
How early can I buy?
Eurostar's usual “booking horizon” is 180 days. If you plan almost six months in advance you can be pretty sure of getting a standard-class return for £58.
But I can't plan that far ahead
Be flexible about when you travel. The very earliest and latest trains are worth checking – with the limited number of onward connections, they tend to be less in demand. Middle-of-the-day services are also cheaper.
I have to leave tomorrow
In that case, you'll probably pay what feels like a fortune. However, you could travel in more comfort in standard premier (effectively first class, with drinks and snacks or meals) for only a few pounds more. See seat61.com for more tips. And there are still many flights between London and Paris every day, with easyJet from Gatwick and Luton often cheapest.
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