There are a small number of museums open on Monday, but you're right that the most important ones are closed. Brussels is very "walkable" from the Centrale stop, so you'll use your Eurostar ticket (each way ... ) to take the included LOCAL, very frequent long-distance rail (NOT the Metro) from Brussels Midi to Brussels Centrale.
If we round everything DOWN, it's two hours to Brussels, and two hours to Bruges. That's two hours back, and two hours home to London. Do you really want to go to Bruges that much? Have you looked at Google Maps? I'd forget about Brussels if you go to Bruges. And the scenic part of Bruges is not around the train station, it's a short local bus ride away. So you have less than two hours in Bruges.
I personally would only do this trip as an overnight, even if it means you pay for two rooms that night, one in each city. Honestly, I've only had waffles in Antwerp, and the big chocolate companies have stores in all the Belgian cities. The question is where can you get chocolate that was made behind the display room where you buy it in Brussels? I don't know.
I'm not a big fan of Brussels (I prefer Antwerp), but Brussels has plenty of sights for a day or two. Some of the smaller churches are particularly sweet. What month is this trip? Walking around is less fun in the rain and wind. Remember that Eurostar is MUCH cheaper bought three months in advance, but then it's non-changeable, non-refundable. Remember to allow time (at least one hour, they'll tell you when they issue the tickets) for security and Immigration at EACH END of the Eurostar trip. That may increase after Brexit is complete, unknown today.
Read up on dishonest business practices on the otherwise lovely Rue de Bouchers restaurant strip. No personal experience, except that I turned back the agressive touts, as I walked. Perhaps this statement is a bit naive on MY PART, but I find Brussels "touristy-er" than Antwerp or even Bruges.
My favorite museum is the dual (Modern/Ancient) art museums (closed Mon). But the Bozar is quite nice, and I think it's open on Monday. The palace cellars (Coudenberg Museum) is unusual, and good in bad weather, but not very meaty. I liked BELview, but it's also closed Monday. It didn't pull any punches about Belgiums Colonial history.