Upvote:9
Cheap flights from Brussels that actually consist of a flight from Amsterdam, Paris or London with a transfer from Brussels are common. There seems to be competition to drive Belgian passengers to these other airports. It does end up with ridiculous situations where it's cheaper to buy such a βflightβ from Brussels plus a train ticket from London/Paris/Amsterdam than to fly directly from the other city.
The airlines are aware of what they're doing and they do their ticketing in Brussels. You won't be able to get onto the plane in London if you didn't board the bus or train or plane in Brussels. You might get away with it on the return journey if you have no checked-in luggage.
Don't you love market pricing?