A couple of points here Guffer ......
The way that plasterboard/stud walls are constructed is that there will be horizontal braces fitted between the verticals. I think these are called "noggins" and there should be a couple of these within the height of the wall. They wont be in line with each other - but staggered to give extra strength. That means that any cable dropped from above will meet a Noggin within a couple of foot from the top.
All the upstairs stud walls I've seen have been built on top of the floor - that means that apart from the lower 4" x 2" of the wall frame there will then be the floor/ceiling before the lower wall starts.
I'm not in the trade, but have run lots of cables around houses - both as add ons and in new build where I had access to the inside of the stud wall prior to the plasterboard being fitted.
Another technique see's plasterboard bonded to a solid wall using dabs of material - this leaves a gap of about 1cm which can then be used for cables - with a bit of luck of course. Also hoping that nobody then hangs a picture and pierces the cable with a nail!
No easy answer I'm afraid as each installation will have its own foibles to contend with
Good luck with the project ........