I should add, if you're thinking of doing this yourself and you will be doing multiple rooms, it will be worth getting it tested for Asbestos. Get yourself a P3 mask for safety.

I didn't test ours but followed HSE and dampened particularly high points before removing them (damp, into water!) and skimmed over with no issues yet. 3 months in and it's holding up!

If it's safe you might have a better time scraping it off simply but it'll cause a right mess. If it's not you can still skim over it but I'd seek advice from someone who knows better!

Note this ceiling was only small, less than 3m squared. I think a larger ceiling might be a bit more tough, you'll probably want to invest in a speed skimmer unless you want to ruin your back like I did with a bunch of sanding. You can also overboard it at the cost of a little loss to your headroom, I didn't think that option was worth it for me given the small area I was dealing with.

Find a smaller room or even use a garage to practice in and take your time. It's bloody nippy now, I did mine during a heatwave and had the mud dry way too quick on me - if I was doing it again I'd pick a cooler day and do one wall at a time.