Get the plane. You will be outside the terminal in Antwerp airport ten minutes from the plane stopping maximum. Buses take fifteen minutes to the centre, or get a taxi. Fantastic restaurant at the airport too.
You will also avoid the queues and scrums at St Pancras; and, especially, Brussels South, which seems to get worse every time I use it. At Antwerp airport you can check in 30 minutes before take off, it used to be less with just hand baggage. It broke my heart when VLM stopped flying from Manchester.
The imponderable is London City: I haven't used it, but it would have to be spectacularly bad to equal the queues at Brussels.