I think you have to book two singles.
You can book Brussels to London on the UK site, but it's immensely cack handed. Select London St Pancras in the To field for a one way journey, then Paris and Brussels become selectable in the From field. An appallingly bad piece of website design which surely must be losing them sales to airlines.
I occasionally book Brussels to St Pancras (return or single) and I have never thought that there was anything different to when I book starting in London. The From field allows me to choose St Pancras or Paris or Brussels without entering anything in the To field, and if it didn't in the past I would have remembered.
I always use the UK site because the price in euros seems to me to be significantly (in percentage terms) higher than the current exchange rate says it should be, and this was the case even when £1 was €1.35 or €1.03 (as it was in 2008). I feel that even cardholders in the eurozone might save by booking on the UK site if their card charges low or no forex fees.
(A similar thing happens when booking Norwegian Air, they allow payment in any currency out of those used by the countries they serve, and the exchange rates sometimes do not match current reality. I once saved 25% by paying in SEK compared to GBP for a flight that didn't touch Sweden, or the UK.)
The only annoying thing I find about the Eurostar website is that when you want to restart your search due to timeout, it doesn't remember your previous search.
Does anyone know whether: if you just buy a return from London to Amsterdam, on your return leg Eurostar would just let you board the train at Brussels Midi?
Given that return from Amsterdam involves getting an intra-Schengen Thalys (or possibly eurostar) and then going through passport control and security in Brussels Midi?
Or do they have some way of telling whether you've boarded in Amsterdam?
I read (possibly on this forum) that any valid Eurostar barcode opens the gates at Amsterdam Centraal.
404250 implied that a Rotterdam ticket worked. I wonder if a Paris ticket would work.
My one-way ticket StP>Ams let me re-enter the station 2 hours after I exited, which surprised me.