I have a rule of thumb when I collect people from Heathrow. Other than watching my app to make sure the flight is on time, I make sure that I am outside the frosted glass doors 90 minutes after scheduled arrival and expect to see whoever I am meeting between 90 minutes and 2 hours.
That gives time for the plane to make the long journey from touchdown to the jetway or the stand and the bus, disembarking, making the long march, queueing for immigration / border control, collecting your luggage and walking through Customs and through the frosted glass doors. Note that Customs is the least of these, a simple walk through the appropriate passage and door unless you have something to declare or you get pulled out of line (rarely) by the invisible people behind the one way mirrors or monitor screens.
I've never had anybody through the frosted glass doors before I got there. I have had a few later, and one nearly 4 hours after landing - but that was when there were well publicised delays in the airports.
So, if your flight is on time, 2 hours for that, 30 minutes to get to the Underground, get your ticket or Oyster, and wait for the next scheduled tube train, and 60 minutes on the tube. Allow at least one hour for Eurostar check-in. So - around 4 and a half hours. It's a shame you can't fly straight to Brussels.
I know you will be on a weekday, but my comments are also for other readers.
With your arrival around noon, the first train I would contemplate, if I were not averse to risk (but I am) would be the 17:04, arrival in Brussels at 20:05. That one doesn't operate on Saturdays. The 15:04 is just too risky. The 16:04 on Saturdays ... maybe ... if ... you want to roll the .... dice. The 18:04 (which is an hour later on weekends at 19:04) is much less stressful but doesn't reach Brussels until 21:05 (an hour later on weekends) and you'd need a late train to Brugge, and probably a taxi there because of the lateness of the hour.
I'm sorry I can't be more helpful or positive.