I don't think you or I know enough about the NHS to have sensible suggestions for their efficiency savings. But I do know this is not a good one.
Plastic surgery is mainly about fixing birth defects, cancer damage, burns, and other disfigurements. I think you'll agree these are good things to offer people for free. Maybe you're thinking of cosmetic surgery? The NHS doesn't (usually) offer cosmetic surgery.
As for weight loss, it's a bit like trying to prevent people smoking. Spending money on stopping people smoking is great value as it saves money later on rather than dealing with complications (cardiovascular disease, cancer). Same thing with weight loss (diabetes, amputations, blindness).