Okay. Wow. Everything I'm doing is wrong. Again. It's just like talking to the missus!
I was planning to use:
12.5mm board on the stud walls - leaving 5.5mm skim.
9.5 mm board on the dot and dab - leaving 8.5 mm for dab & skim ... so perhaps a bit tight in places but certainly not over generous.
I was aware of the two reasons for board not touching the floor but the work is on the second floor so no real damp issues - I was not sure about the expansion issues in this specific context, specifically whether a gap should be between board and base, under the base or both. ... Hence I asked the specific question that I did.
I want to attach the skirting boards with screws hidden with wood plugs.
Batten would certainly be a cheaper option. Were it not for the fact that I have a table saw and a couple of sheets of 18mm MDF that are currently in the way .... and I would have to buy battens. Don't think "free" solution versus the "paid for" solution is the more expensive one ... is it?
Fixing two battens to a wall doesn't seem to be much, (or in fact any), easier than similarly fixing one MDF board.
There are other idiosyncratic aspects to this specific job, completely unrelated to gaps at the bottom of plasterboard that make the marking of the location of battens for use after skim coating in some places, really quite difficult.
I am reminded of an old joke about a man winding down his car window to ask a passer by the way to Bodmin. The passer by strokes his chin and says "Mate. If I were going to Bodmin, I wouldn't be starting from here".
What happened guys?
In the old days specific questions got specific answers if anyone knew those answers. Increasingly the answers are all "you're doing it all wrong ... stupid!".
Everybody here knows, or ought to know, that with all jobs "context is everything". Why then is there always an assumption that the questioner is a complete idiot unless he first writes a novel explaining every single detail, relevant or not, of why he is asking the question in the first place?
Do I need gaps under the plasterboard / baseboards? That simple question was what I asked. I have never come across this problem before in forty odd years of doing DIY.
Thank you for all your answers. Even though none actually responded to the actual question.
I'm sure I'll figure it out.