Doctors, nurses, consultants and the NHS in general, treat us with our permission. They have no powers to make us accept treatment if we object.
If you are happy with your weight loss (and at 4 stone in a year, you should be delighted), and are willing to carry on (if you want to lose more weight), then simply ring up and cancel the appointment.
It would probably be a good idea to write a letter too, clearly explaining that with your family history of slim diabetes, and the great success of your current diet regime, you don't feel that you can commit yourself to another year long regime, with the prospect of surgery at the end. If things change in the future, you will be willing to reconsider, but in the meantime, you would like to continue with your existing diet and diabetes regime. And please can you go back to your normal schedule of clinic appointments?
Send a copy of the letter to your doctor, your consultant and the fat clinic, breathe a sigh of relief, and wait for the next normal clinic appointment to arrive.
The odds are, you will see a completely different consultant or registrar next time, and the subject will never come up again!
The whole point of the letter is to avoid the black mark of 'non compliance' from appearing on any of your records.
Good luck!