Your split tickets work exactly the same way as any other Tir-Phil to London train tickets on the National Rail network do.
Instead of having one train ticket to take you from Tir-Phil to London the journey is broken down into two or more parts with a separate cheaper ticket for each sector where your train would ordinarily stop. You are still able to reserve your seat, enjoy additional discounts if you have a railcard and you do not have to change trains at the stations where you have split your ticket. For more information on Tir-Phil to London split train tickets follow the links in the menu bar below.