I'm not sure I'm following the other advise though, are you saying to chop out the plaster from behind the skirting or the bottom of the skirting board?
Ideally, you need to install skirting board as plumb (vertical) as possible, so that it looks right and so that you don't end up struggling with the internal corner scribes. You also want the gap, if any, between the top of the skirting and the plaster to be no more than 3 or 4mm. Finally, you need to support the skirting board when it has been installed so that it stays put if someone accidentally kicks it.
As far as I can see your wall has two separate, and fairly common, issues. Firstly, the bottom of the plastered wall "kicks out", thus (exaggerated for effect in Dwg A):
It is this "kick out" which needs to be got rid of - and for that I tend to be quite brutal and fast by using a club (lump) hammer and an electrician's bolster chisel to pare back the plasterwork
below the top of the skirting level. It doesn't need to be pretty because it will never be seen, but the end result is that you should be able to get your skirting tight in to the plasterwork, or at worst to within a few millimetres (as in Dwg B, above). That small gap can then be filled with decorator's acrylic caulk
I have some low expansion foam there... I'll add some more pics to show the full skirting length.
The second issue is that you have a missing strip of plaster at the very bottom of the wall (see Dwg B, above). You could fill this, but it's often a waste of effort, and in rooms like bathrooms the result could be water wicking up the plasterwork in the event of, say, a bath overflowing. So a better way is to fill it with something which will both fill the gap and stick the plasterboard in place - a grip adhesive or a
low expansion adhesive foam (for anyone else reading this who isn't familiar with it, which is NOT the same as expanding foam).
If the wall has an end to end bow in the wall you'd normally need to fix the skirting to the wall in some way starting at one end and creeping along to the other, In trade work we often use a 2nd fix nailer for this, providing there is something to fix to (even 16 ga nailers won't go into solid brickwork). Fixing with foam you can do this by simply putting weights (e.g. blockwork wrapped in cardboard, timber battens screwed to sub-floor, heavy books inside a plastic bag - but no first editions unless they're some of Mad Nad's!, tool boxes, etc) which are left in place until the foam has set.
