Whereas the majority of passengers travelling on the Eurostar are British citizens, who do not need a visa.
On Paris trains most people seem to be British, but there are significant numbers of French nationals and people from all over Europe and the world. London/Kent - Brussels passengers mainly seem to be British, Belgian, Dutch or German nationals. London/Kent - Lille passengers mainly seem to be British nationals with a few French nationals. Lille - Brussels passengers are presumably mainly French or Belgian nationals.