My knowledge of our build is that we were actually not permitted to visit the worksite (our house) until handover or a visit with the supervisor. Even though we did.. So, if I was to bring up the fact (if I had of seen it back then) that the walls are crooked, wouldn't they have asked "why were you there?" & brought up some legal jargon?
I am not better informed with most things now regarding the build so the quality of my (maybe) next build will be 'should' be quite superior.
It’s probably not the plasterers fault but the carpenter who constructed the frame and the carpenter who was supposed to do the internal fix by planeing or packing out bad walls. Either way, someone didn’t do their job and you are left disappointed. The only way it would be the plasterers fault would be if they hung a sheet of plaster with an obstruction in the way (pine shavings etc.) that prevents it from sitting flush to the studs. Keep in mind that the metal trim they put on external plaster corners is not a perfect right angle but a more acute apex and this can make walls look crooked, especially if the wall runs close to a grout line or if it is a nib wall. Though that does not look to be the problem in your case as you have a bow in the middle of the wall! Other trades that work over the top of existing defects are not paid enough to stop work and bring it to the attention of the SS- even though they owe you a duty of care to do so.
You can do two things to fix this. If the problem is not that bad, remove the skirting and have a plasterer ‘float’ the wall out and reattach the skirt and repaint the surfaces as necessary. No need to cut the tiles to refit the skirt- just have the skirt trimmed and placed on top of the tiles and use silicone matched to your grout in the join. Or if it is too much for the plasterer to float out, then you would need to cut out a section of plaster bigger than the offending area and pack out or plane back the studs to bring the wall to square, level or plumb- depending on what the problem is.
I am doing my own electrical in the house so am allowed to be on site whenever I want. Though I can’t imagine a contract would stop me going on site to inspect my own home in any case- I would just have to be very creative to justify doing so!