London to Bruxelles / Brussels by train

At St Pancras:

Eurostar is currently suggesting that Standard Class And Standard Premier ticket holders should be at the Eurostar departure area in St Pancras International 90 mins before departure; and that Business Premier ticket holders should be there 45-60mins prior to departure.

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.

Our guide to taking a Eurostar from London is here.

Note that London is in a different time zone to mainland Europe, it is one hour behind; hence what can seem like a discrepancy between the journey time and the interval between the departure and arrival times.

Arriving at Bruxelles-Midi station

If your train will be departing from London at 06:16; 08:16: 11:04 and 18:04 /18:13, it will on from Bruxelles to Amsterdam, so it will arrive in the part of Midi/Zuid station that is also used by ICE, Thalys and TGV trains
Trains departing London at other times terminate in the dedicated Eurostar arrivals area at Midi/Zuid station.

Book early and save: Yes

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

Also note that on the Eurostar booking site, it only uses the English name of the main station in the Belgian capital - so enter 'Brussels Midi' into its search.

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'

Eurostar train's general ticket terms:

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 a return to the UK on the final day that you will be using a pass.