Of course. Interestingly, just about half of the nearly 6m UK - Paris / Brussels air passengers* travel to/from Heathrow, Gatwick and Luton. (The numbers from City and Stansted are negligible). In addition there’s about 9 million on Eurostar. Taken together, that means that only 20% of all the UK - Paris / Brussels air/rail market does not use London. So whilst there are large areas of the country where it doesn’t make sense to use Eurostar, in terms of numbers of people in the market it is a relatively small proportion.
Back on the subject of Amsterdam, it is a similar picture. 46% of UK - Amsterdam air passengers travel via a London airport.
* some of this will be interlining at Heathrow or CDG and therefore not a true alternative to Eurostar.
Source: CAA route analysis 2017
I'll be travelling from Brussels to Nottingham soon and will be getting the Eurostar instead of flying to East Midlands Airport just because it's easiest. That's a slightly special case though as it's on the Midland Mainline and so a very easy change from St Pancras. But total travel time isn't too different, and getting to Brussels airport is quite expensive these days thanks to the Diablo surcharge (it's actually cheaper to get a train to Antwerp airport from Brussels than Brussels airport).
I would expect most people flying to Brussels from London would probably be changing planes onto somewhere else. Brussels airport has some aspirations to be a hub - though slap-bang in the middle of Schipol, Frankfurt, and Paris CDG it is in a crowded field.