Lead paint was banned for residential use in 1978 in the United States and as late as 1992 in the UK, so any house that was painted previous to that date could be lead-based. If sand it becomes a dangerous air born particulate. If inhaled and absorbed into the blood stream.... Well, we all know it's bad for you. If you are unsure there are test kits for lead in paint: https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=Lead+paint+
After donning protective gear (respirator [filtered type], disposable suit, etc.) scrape as much of the offending paint from the wall as possible. Use a wide head scraper with a handle above the scraper head which will enable more force to be exerted onto the sharpened steel thusly removing more paint faster. Try to use sanding as a final, desperate act. If you do need to sand you will need to make safety accommodations for the dust: plastic tarps to enclose the area, window fans to exhaust wayward dust, vacuums attached to sander exhaust ports. And the tedious clean-up of the surrounding surfaces when sanding is done for the day. Place all paint chips and any disposable gear in a plastic bag and tape it closed. Bring it to a local hazardous waste collection site.
Wash the newly exposed walls with a solution of TSP and warm water. Once the freshly washed walls have dried sand with 80-100 grit paper, wipe the walls with a damp rag. Prime and paint as usual.