just had a look on the website and it is clear on there that you can use your eurostar ticket to continue on to Bruges.....
Explore Belgium
Brussels isn’t the be all and end all of Belgium you know. Your Eurostar ticket will take you further, much further – to Ghent, Bruges or Antwerp, for example.
Buy a Eurostar ticket to Any Belgian station from as little as £80 and you can travel on to any other Belgian city at no further cost.
Just get off the Eurostar at Brussels-Midi, change platforms, and get on the train to one of these great destinations. Just show your Eurostar ticket to the nice man on the train, and you’ll be on your way.
I assumed the bit in bold meant you had to select the option for any belgian station . Surely if not it would be pointless having that option on the site