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Unlucky. That's a lot of work however you approach it. Time wise it would probably be better to strip the whole lot off plaster included and start again from bricks or stud with new plasterboard or render and plaster, that's what a builder would do.
Just estimating the alternative in time and materials and it's a lot. Is it possibly 'Artex' since that was used to cover all manner of problems.
Looks like plaster over paper to me.
I would remove everything that wants to come off.
Big two handed scraper with replaceable sharp blade.
Then prime with SBR or PVA before re-skimming.
It’s a job for loud music and your stimulant of choice.
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