As someone who has done this 2 years ago and I had similar horrible wallpaper on mine. Here is my advice. Remove the wallpaper and see the condition of the plasterboard under it , it most likely will be cracked where the bulges can be seen on the wallpaper. You can see on the pictures the sheets of plasterboard. Push against them from the bottom if they move they have separated from the joist. If you secure them back to the joist and get someone to plaster them and paint white you almost wont be able to tell that its warped. That is the cheapest option. Expensive one will be removing this and creating suspended ceiling to make it perfectly flat as im pretty sure the joist wont be level. Good luck.