Hi Lovely, I was also worried about this before deciding to go ahead with my BA as I’m very petite and weigh just under 7 stones. I dont think they would ever cancel your surgery on the day due to being underweight as this would be picked up at your pre-op generally around a week before so if they have concerns about it then they will advise you before surgery. I think they work it out on whether you have a healthy BMI as everyone is different. Hope that helps xxx