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.

The SMTJ guide to taking a Eurostar from London.

Checking In:

Eurostar is currently suggesting that Standard Class And Standard Premier ticket holders should be at St Pancras International, ready to board. 90 mins before departure; and that Business Premier ticket holders should be there 45-60 mins prior to departure.
This advice does not mean that you should be generally at St Pancras International station, this much time ahead of departure - and therefore can use this time to have a meal or shop.

The departure gates will close 30 mins prior to departure for Standard Class And Standard Premier ticket holders; and 15 mins prior to departure for Business Premier ticket holders.

It is up to you to allow time to pass through check-in etc and board the train, though teams of staff will aim to ensure that you won't miss the train if time is tight.

The Eurostar timing recommendations are in place because many Eurostar departures are close to being fully booked - and when that is the case, it typically takes around an hour for all passengers to be processed pre-boarding.
Queues will inevitably form to access the check-in per departure and it can be closed before all of the passengers for that departure have been processed.

So when you first arrive at the station take a look at the queues, or ask the staff what they advise.
Your priority when you get to the station isn't to relax and have a drink, something to eat etc - Instead it needs to be 'how long is it going to take me to pass through check-in and be in the departure lounge'.

Book early and save: Yes

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

Flexible tickets

On Eurostar trains to and from London, the only ticket type are the Flexible tickets, but it is the class of service you opt to travel by, which impacts on the terms and conditions of travel.

*Exchanges

Book to travel in Standard and Standard Premier class:
The booked date and time can be exchanged to a different departure to the same destination up to one hour* before departure time.

• If the cost of your new ticket is more than the cost of your original ticket, you will be required to pay the difference;
• If the cost of your new ticket is less than the cost of your original ticket, you will not receive a refund of the difference.
  *Note that on you day of departure, if your journey to St Pancras is being compromised and you know that you won't make the booked train in time more than an hour ahead - Taking time out to access the service you booked with and making an exchange, will be a cheaper option than having to re-book.

Book to travel in Premier class:
The booked date and time can be exchanged to a different departure to the same destination up until 60 days after your booked departure date, without having to pay any price differences on any other charges.

The Eurostar ticket terms summary is here

Refunds

Book to travel in Standard and Standard Premier class:

• Up to seven days or more before the date of departure you will need to pay a fee of £25 per person for a one-way / single ticket and £50 per person for a two-way / return ticket - So you will only receive any sum you paid in excess of these sums.
• Less than seven days before the date of departure, tickets are non-refundable - Also applies if you exchanged a ticket to an alternative departure, under seven days ahead.

Book to travel in Premier class:
Up until 60 days after your booked departure date, tickets can be refunded at no additional cost

The Eurostar ticket terms summary is here

Seat Reservations

Reservations are mandatory so seats are assigned when booking - But after a booking has been completed, you use the ‘Manage Your Booking’ facility, which can be easily found on the Eurostar website and select specific seats within the travel class you have chosen.

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

You don't need to have booked the ticket with Eurostar
You will need to enter a Booking Reference number - but other booking agents such as Trainline, Rail Europe, SNCF Connect, B-Europe and NS International, will also include this number on the communication sent to you.

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 with Rail Europe - see this guide on how to do this.
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.

Or 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.