Eurostar St Pancras - Brussels takes about two hours. But you've got the walk/taxi down Euston Rd off the Manchester train, then half an hour check-in. I go from Doncaster-Brussels or Antwerp a lot and the times by train/air are very similar. But if the price is OK I prefer train to MAN then Cityjet to Antwerp as you can be off the plane and outside in five minutes - no exaggeration.