I truly think that Eurostar is the most convenient way to travel from Brussels to London. No need to convince me, I totally agree. But I can tell you from my experience that when you travel last minute like I did for 1 year that Eurostar is often the most expensive. This is true that you have to pay for the tube to go to LHR, but depending where you are living in London, it can be sometimes cheaper to go to Heathrow than St Pancras.
By the way one day I was OFF and so in Brussels, Germania called me to see if I can operate a Gambia Bird Airlines flight from LGW to FNA (ow nostalgia) because of a sick cabin crew, I agreed but told them I was in BRU. They made me proceeding to LGW to operate the flight and they decided to get me on a Brussels Airlines flight to Heathrow and then with the train to LGW rather than with Eurostar to St Pancras. So I’m not the only one thinking this way
But I guess If you have the chance to book your ticket well in advanced the Eurostar is maybe not so expensive, but I only hav’d this opportunity once