London to... Bruge? Brussels? Antwerp? Rotterdam?

You could do Calais to Brussels in a day, it's only 200k. I did Dover, Calais, St Omer (V2 rocket site), Lille and Villenueve d'Asq are a bit shit, Mons and Charleroi I don't recall, Tournai was nice or you could go more direct through Ieper/Ypres is an important WW1 war memorial town (Menin Gate, trenches), Kortrijk/Cortrai (i can't recall but I went through there).

If you end up choosing between going through Mons/Charleroi/Tournai etc and Ieper/Kortrijk, I would definitely go with the ieper way. Those areas are just much nicer places, pretty/interesting stuff to see etc.
Having said that, I've never been to any of them on a bike...