I had to smile when I read what you wrote here and I am sure you must
know more than I do Paul BUT every time I have traveled
London/Brussels/London with the exception of once two coaches would
have been sufficient. I usually travel outward by the 15.40 on a
Tuesday and return Wednesday by the 14.59 the first time I traveled it
was on a Monday afternoon and the train was heaving .

The reason the tickets are cheaper Tuesday and Wednesday afternoons is
because these are the least crowded services.
--
Michael Hoffman

Looking at the eurostar website it seems the cheapest tickets for this
Wednesday are £120 each way to either Brussels or Paris, even though I
expect there will be huge numbers of empty seats. (easyjet in
contrast are charging about £130 return to Paris). When I went to
Brussels a few months ago there was only one other passenger in my
coach, but tickets would have been over £200 return had I not bought a
youth fare. Eurostar's ticketing seems like BA's used to be - gouge
those travelling on expenses, and don't bother chasing those on a
budget. This is in contrast to, say, the TGV (where most fares are
reasonable) or Ryanair (where the prices start at zero and are managed
carefully to get a very high load factor.)