Eurostar used to offer through tickets that were valid to "any Dutch station", but now you can only get through tickets to/from Amsterdam or Rotterdam via the Eurostar website or app.
You can book through tickets between London and any other Dutch station using the NS International website or app, but you have to specify which Dutch station you are travelling to and from whereas you used not to have to do this with "any Dutch station" tickets.
I would presume that if you are returning from a different station in the Netherlands to where you travelled to on your outward journey, your best option would be to buy a separate ticket to Brussels from where you are travelling from on your return journey, then using your existing Eurostar ticket for the Eurostar leg of the journey even though you won't have used the leg of that ticket to Brussels from the Dutch station that you travelled to on your outward journey. Obviously, with the now withdrawn "any Dutch station" tickets you would not have needed to do this.
I may be making such a journey next year, going to stay with friends in Dordrecht and then going to a festival near Boxtel which is where I would be travelling back from on my return journey.
I presume the Eurostar check-in gates at Brussels Midi will accept through tickets from any Dutch station or from Cologne, as many such passengers will be changing in Brussels.
You can book through tickets between London and any other Dutch station using the NS International website or app, but you have to specify which Dutch station you are travelling to and from whereas you used not to have to do this with "any Dutch station" tickets.
I would presume that if you are returning from a different station in the Netherlands to where you travelled to on your outward journey, your best option would be to buy a separate ticket to Brussels from where you are travelling from on your return journey, then using your existing Eurostar ticket for the Eurostar leg of the journey even though you won't have used the leg of that ticket to Brussels from the Dutch station that you travelled to on your outward journey. Obviously, with the now withdrawn "any Dutch station" tickets you would not have needed to do this.
I may be making such a journey next year, going to stay with friends in Dordrecht and then going to a festival near Boxtel which is where I would be travelling back from on my return journey.
I presume the Eurostar check-in gates at Brussels Midi will accept through tickets from any Dutch station or from Cologne, as many such passengers will be changing in Brussels.