According to the National Institute for Health and Care Research (NIHR) the number of NHS knee replacements has fallen in those areas which have introduced rules on weight and body mass index (BMI). The rates of surgery fell in all patient groups, not just in those who are overweight.
This insistence on weight loss (and physiotherapy) before a patient is even considered for knee surgery seems to be short-sighted and little short of rationing. It leads only to further health inequalities: the affluent are paying to have the surgery privately but the economically deprived suffering major impacts to their quality of life and ability to work.
NIHR press release at https://www.nihr.ac.uk/news/knee-replacement-surgery-decreased-after-nhs-policies-on-patients-weight-were-introduced-worsening-health-inequalities/30913.
[Thanks to Ewa Siwiec for finding this information]