I think this isn't quite true - in the SE a lot of rolling stock (most of it relatively new) is sitting around between the peaks - take a look at Camden Bank at noon, for example - whereas in the North there's relatively little peak strengthening/frequency increases so there isn't that big cost.
Thameslink is a bit different due to the long fixed formations, but a 12-car at 2pm is going to be carrying a lot of fresh air.
I was more thinking about infrastructure, but even with the trains many of the SE trains which stable in between the peaks will manage two productive peak-flow trips, and between the peaks also allows an excellent opportunity for maintenance. Take the GN 365s as they are now - relatively low mileage, electric, 100% DOO, every single diagram stables at the country end overnight, and goes into the maintenance depot between the peaks.
For something like Northern one could be doubling or even trebling the fleet size just to provide capacity for one morning and one evening trip, perhaps for a section as short as Apperley Bridge to Leeds. Meanwhile GN might have a 12-car train with people standing Cambridge to London.
I’m not quite so familiar with the LNwR setup, but are your lines of trains in Camden sitting there because the operator is content to see people stand on 4-car trains during the daytime because they don’t want to pay the extra running costs?
