It works both ways, SP aren't at a complete disadvantage. For sure, there probably isn't much money in foot passengers - but vehicles I'm guessing are another story. Putting everything in containers straight for Mezeron is a disadvantage too, when they'd normally be just on the road network in the UK. The passenger facilities on the SP are primarily needed for vehicle drivers, commercial or non-commercial, and foot passengers are just a bonus on top of that.
As a going concern, SP itself is still profitable (still even a decent profit, the pre-Mez profit was just bloated).