The 25% discount is valid only when booking Advance train tickets used on Grand Central train services. The discount is not available if you are travelling with any other train operator, even if you book this via the Grand Central website.
If part of your journey is with Grand Central and another part with another train operator, then you must buy a valid ticket for that part of your trip. All Advance tickets are subject to availability.
Check out our popular routes page to see where you could be travelling to next with Grand Central.