Fwiw, we did the math ourselves when living in London: Eurostar to Paris was a no brainer time saver, as was (pretty much) Eurostar to Brussels. But we opted to fly to Amsterdam - time lost at the airport was gained by the short flight.

This was influenced by us living in South London (Brixton). If we'd been closer to St Pancras / further from the airport, the math may have differed.