Food addictions, intolerances, depression all sorts of things can lead to obesity and whether you like it or not the NHS becomes responsible for the healthcare of these people. Now if you want to stop the funding for this surgery then go ahead, you'll save the cost of about £8000 per op. Now to flip that over treatments for High Blood Pressure, Diabetes, Joint and muscular problems, skin conditions, digestive tract illnesses etc etc ...that will cost you double per person per year. Been there done that...well the memsahib has.
When the surgery first became available in Scotland the pioneer surgeons near me were given budget for ten of these ops as a study of the benefits to the patients and the saving, long term, that these ops brought about. She was lucky to be involved with the Dietician Prof who pioneered these ops in Scotland and had a band fitted.
My wife weighed 23stone, a combination of depression following the deaths of two of our children and the discovery of an intolerance to carbs which they found at the end of the process. She was fitted with the band and lost around 12 stone over a two year, closely controlled period. She's now at the end of the process weighs about 11 1/2 to 12 stone, is off all of the medication she used to take now just uses vitamins. She wasn't "Fat or Lazy" and the OP should realise there are many reasons that obesity hits people.
On the flip side of that a guy who works beside me had one done, lost all his weight and got some plastic surgery for loose skin, he's now heavier than when he started, he shouldn't have had the treatment and I'd agree that individuals like this shouldn't be funded.
The local trust ensures that the budgets are matching and that savings are made, last year they had budget for around 20 and they have gone to the patients who are most likely to make the best use of them, lose the weight and keep it off. Criteria, even to get referred to a dietician are severe, BMI of over 55 and underlying health problems, its not a give away solution.