I’ve cycled from London to Brussels a couple of times with organised groups on road bikes. We’ve gone via the Dover-Calais ferry, which is usually pretty straightforward, the port staff will direct where you need to go, and you get the weird experience of riding your bike on and off the ferry. From Calais to Bruges is a nice ride, especially if you have the wind behind you, fairly flat and pretty. And Bruges is ace.
I can’t remember a lot about the ride from Bruges to Brussels. Well, that is, I can’t remember much positive. I recall endless miles of obligatory bike paths (where there’s a blue sign, you’re compelled to use the bike paths and aren’t allowed to ride on the road), which were safe but boring, hours of b’donk b’donk b’donk riding on the concrete slabs with tarmac expansion ridges between them. And Brussels itself was underwhelming to arrive into, from what I recall, but then most cities are if you have to ride through the outskirts for an hour or two.
Sorry not to sound more positive! My enduring memories of riding in Belgium are hard, dull, grey, windy and wet… it’s probably quite nice if you get good weather!