Take Eurostar from St Pancras International station in London to Midi (also know as Brussels Zuid) station in Brussels via the Channel Tunnel.
Standard ticket starts at €34.50 one-way and the journey takes about 2 hours 15 mins. Be sure to book your train tickets early as the prices rises when cheaper seats are sold.
To make sure you’ve got enough time to go through the security checks and passport control, its a good idea to arrive at the station at least 45 minutes before your train departs.
If you are picking up your tickets at the station on the day of departure, be sure to leave a little extra time in case there is a queue.
Check in close 30 minutes before your train departs (for Standard or Standard Premier) and 10 mins before (on Business Premier)
Your Eurostar ticket will be valid for travel to and from Brussels-Midi/Zuid and within the Brussels Metropolitan area only. This means that the ticket is valid on connecting local trains to other stations in the central Brussels area, including Brussels Central (5 minutes walk from the historic Grande Place), Brussels Nord, and Brussels Congres.
Check out Eurostar website for train timetables and information on other destinations.
For more information and tips on booking Eurostar trains and on Belgium, check out this great website.