You used to be able to go to France or Belgium at no notice at reasonable fares. If Eurostar was affordable at short notice then we wouldn't be having this discussion. I mentioned earlier that it is still possible to use the ferry as a foot passenger but it is a lot more expensive because of the withdrawal of through tickets, it is less convenient and takes longer because the stations are not near the ports any more, it is less frequent, and there is no longer a night service for foot passengers.
If Eurostar is too expensive then London to Brugge or Gent now involves train to Dover, walk/taxi to the docks, ferry to Calais, walk to Calais Ville, train/bus to Dunkerque, bus to De Panne then train (or train via Lille). Or via Hoek van Holland, Rotterdam and Antwerpen. Or by coach. All these options are far worse than getting the ferry to Oostende.