Flying to London

Don't fly. Take eurostar. Faster and you are in the centre of town straight away.

"Sometimes it is expensive."

Both Eurostar and airlines use market-pricing in attempt to maximise their revenue. That means that prices on both are highest at popular times and on popular dates (and especially at popular times of the week - Friday and Sunday afternoons, for example) and are usually at their cheapest when bought well in advance. There are, though, almost always some very cheap flights, they tend to be end of day positioning movements that depart or arrive at times when public transport access to airports isn't available - add in the cost of a taxi and they aren't cheap any longer.

If Eurostar is expensive, the airlines, even the "cheapies", are likely to be too, and when you start to add in the cost of getting to and from airports, especially the "geographically challenged" ones that the cheapies tend to favour, flying is rarely the better choice unless your origin and/or destination point is close to the right airport.

@Becasse

Pretty sure EasyJet and Ryanair still fly for about half the cost of the Eurostar.

You'd be a fool to choose either air or train because of a perception of one being cheaper than the other. The sensible person will cost both options, unless time is more important than cost, in which case Eurostar gets from centre to centre faster.
Cheapest Eurostar is 75-80 return.
Easyjet and Ryanair do not fly from Brussels national to any London airport. They don't fly to London out of Charleroi either.

Eurostar: central Brussels to central London
faster than flying (121 minutes)
easier than flying
probably less expensive when all costs are included
less tiring
less wear and tear

From Antwerp (International) airport you can fly to London City airport , not the cheapest option , but the closest solution Londoin end.
BXL to Antwerp (International) airport might be easier than other options to london from adjoining airports.

The Eurostar must be damn good to have unrelated people pushing this hard for it

The best flights, Brussels to London are probably on Ryanair. Drive an hour to Charleroi Airport, wait two hours for check in. Then fly to Dublin. Once in Dublin, take a connecting flight to London Luton (allow at least two hours layover in Dublin). On your arrival at Luton, take a 1.5 hour connecting coach to London Victoria bus station. Low cost, convenient and time effective.

Or of you want to throw away €75, and waste 2.5 hours of your life, take the Eurostar.

Apart from London city airport which isn't really a low cost airport, none of the London airports are central and even London city is on the end of a train line so not that central.
If you have time and want a cheap option the megabus coaches were running last year, but they take hours and aren't reliable for arriving on time plus standing out in a street as they don't pull into a station.
I agree Eurostar is certainly the fast way and relatively inexpensive compared with some flights. Heathrow, gatwick are outer London and stanstead about next nearest but all require transport into central London.
So if you absolutely want the shortest travel time then take the train.