To take a ferry to Dover, especially with lots of luggage, it would be best to drive or be driven to Calais, Dunkerque or Boulogne, and indeed preferably onto the ferry and off the other side. If you want to drive you can check ferries on here: www.directferries.co.uk/dover_ferry.htm
If you want to go by public transport the cheapest way, and provided you can get a departure at a good time, also one of the easiest, would be by Eurolines coach, see https:/…ssl_index.asp.
Depending on your day of departure (I know there is one on a Tuesday), there is a bus at 10:00 from the Eurolines terminal next to Brussels' Gare du Nord (North Station) and arrives in the UK at 14:45 (local time, they are an hour behind the continent). However, I believe many departures are overnight and arrive at Dover at something like 3 a.m. Naturally if you go by bus this includes either the ferry or the Eurotunnel (train) fare, they switch between the train and the ferry depending on which is most advantageous for the coach company.
A more expensive but faster possibility would be to take the Eurostar from Bruxelles-Midi to Ashford (www.eurostar.com), and change there to a local train to Dover Priory, which takes just over 1/2 hour e.g. 16:00 arr. 16:33 (total travel time just under 3 hours). In June this should not be a problem but I have heard a rumour that from the Autumn, when the London Eurostar terminal is moving to St. Pancras from Waterloo International, the stop in Ashford will be dropped. For timetables for your day of departure, go to http://www.b-rail.be/int/E/ and fill in Bruxelles-Midi as departure station, Dover Priory as arrival station and if prompted, VIA Ashford (Kent), written like that.
If by any chance you really want to go on one of the cross-channel ferries and don't want to drive, you could get to each of the ports concerned by train, just about: Calais takes 2h40 and Dunkerque something like 3 hours, in both cases via Lille - but most trains from Brussels, including the Eurostar, arrive at Lille-Europe and you would have to walk to the neighbouring Lille-Flandres station to continue your journey. Brussels-Boulogne takes about 4 hours despite the fact that you have to start by Thalys (high speed train) to Paris-Nord (1h21 mins), but you can then change at the same station for the train to Boulogne-Ville - although there are departures involving changes elsewhere. With luggage, the Eurostar or Eurolines routes would be a lot easier.