How to take the thrilling high speed rail journey from the UK capital to the French capital

Note that London is in a different time zone to mainland Europe, it is one hour behind - so when looking at a timetable the journey will look as though it's an hour longer.

Book early and save: Yes

Online bookings open: up to 330 days ahead of the travel date.

The price shown below is the standard, most heavily discounted, one way Standard Class price; however, Eurostar frequently offers special fare/deals promotions, so the price you see on the home page of the Eurostar website MAY be cheaper.

Another big plus of booking the Eurostar direct on Eurostar.com is that you can access a 'Manage Your Booking' facility - which enables you to change the seat that you will have been initially assigned when making a booking.
Click on the 'train guide' for more information on how to do this

As the fabulous Seat 61 explains here in detail (scroll down to 'Choosing A Seat') some seats on Eurostar services are better than others.

The different types of ticket summarised

Also worth knowing is that when travelling by Eurostar you can choose between three classes of service, and this also impacts on the exchanges and refunds of what Eurostar refers to as 'Flexible tickets'

Customers can exchange tickets for travel in Standard and Standard Premier class multiple times without paying an exchange fee, as long as tickets are exchanged 7 days or more before the departure time of their individual journey
Though if the new ticket option is more expensive the difference in price will need to be paid, but if it is cheaper the price difference won't be refunded.
But make the exchange within 7 days of the individual journey and an admin fee of £30 will need to be paid, along with the price difference
Tickets to travel in Standard and Standard Premier class are not refundable.

In contrast tickets to travel in Business Premier Class can be refunded and they can be exchanged to alternative departures without paying the admin fee, or the price difference.

Eurostar's full ticket terms and conditions are here

Using Rail Passes on Eurostar:

Rail pass users can now pay reservation fees to travel on Eurostar trains, in a similar manner to other international European high speed trains.

The new rail pass reservation fees are similar to the cheapest single journey ticket prices, but as those cheapest tickets can sell out quickly, it's likely that the rail pass reservation fees will save money.

You can purchase these online with no booking fees via B-Europe - or they ARE now available on the Eurostar website.
Booking in advance is recommended as it's not unknown for rail pass reservations on Eurostar services to sell out days or weeks in advance, there is evidently limited availability per departure.

You can obtain a rail pass reservations without paying a booking fee at Eurostar stations.
However, if you want to do this on the day of travel be prepared to be flexible re: the train you will take.

If you are a UK resident, your rail pass + reservation is only valid if you take a train from the UK on the first day that your pass is valid for and return to the UK on the final day that you will be using a pass.