I'm kind of excited because I just watch an obesity documentary about the NHS discriminating against the obese, so I knew all the tiers!
Get the surgery. First of all, you are never going to get this chance again on the NHS. You are lucky to be this close now. You will never be any younger than you are right now and the longer you wait the more damage you do to yourself. So unless you think you can afford to self pay at some point in the future, get surgery now when it is available to you.
Secondly, every single day you are obese you are straining and damaging your body. Sure you can lose on your own, we all can, but can you keep it off. Forever, easily. Even people that regain after losing, usually don't regain it all. Even if they do, the time they were smaller was still a break to their body. Your other medical issues would be resolved in a couple months.
My diabetes was well controlled with a couple pills a day and a low carb diet. Within 2 months of surgery I was completely off meds.
I wouldn't roll the dice with the NHS, if you can have surgery now, have it, you won't get the chance again.