can't remember. I guess that sncb tickets would be in either French or Dutch (Bruxelles-Midi/Brussel-Zuid, possibly abbreviated to Bruxelles-M//Brussel-Z), sncf tickets in French and ns tickets in Dutch. Don't have a UK-issued eurostar ticket at hand so not sure what the story is there.

My theory why Brits don't use Brussels South is probably a reflection of limited linguistic knowledge/fear of confusing their interlocutor. Most Brits, even monolingual ones or with only schoolboy French at their disposal, will guess that Bruxelles-Nord or Brussel-Noord would be the same as Brussels North. Ditto for central/centraal. Some might have more difficulties in making the connection between Midi/Zuid and South. So using the original version might be less confusing.

Strangely, though, whereas Brussels-North is widely used, many (most?) English speakers would say "Gare du Nord" rather than Paris North Station.