All the following is with full respect AndyJ. The following is intended to help motivate you a bit.
I’ve had 5 knee operations in the last 5 years - one of them being a partial knee replacement - so I know a little about how painful and debilitating it can be as I could barely walk, but the bottom line is that you have to decide you want to do something then find a way to do it without letting anything get in your way. I found a way to get back to competitive soccer even after ten surgeons told me I’d never run again. I can still run a sub 5 second 40. You can do anything you want to do, you just have to not accept any excuse or reason why it can’t happen - that’s the hardest part right there.
For exercise, you can do a lot without knees and I suspect that if you can get your weight down, your knees will feel a lot better. You could get a set of 3lb dumbbells, or whatever weight, and do lateral raises, overhead presses, curls, tricep kickbacks… the list is endless. You can do low intensity abs work in a variety of different ways. You can still work legs without knees working well - side steps with bands, standup and sit down 50 times a day, use a lot of ice. Search up some YouTube knee rehab exercises - there are plenty of ones that I suspect you can handle - then do them lightly - like PE, this is going to be a long process.
Just get your heart rate up a bit each day and burn a few extra cals. It also has the addictive side effect of releasing endorphins which will make you feel better all the way around. You’ll sleep better too. I use Fitocracy to track all my workouts - it’s a buggy PoS, but it has a social aspect and it helps to motivate me.
You mentioned earlier that you tried a dietician, but it didn’t work. Why? Did they put you on an unrealistically restrictive diet for example? It all comes down to burning more calories than you take in - which I fully understand can be extremely painful. When I’m dieting (like now) and on a heavy restriction of some sort, I rely on water to make me feel fuller - not the same as ice cream or ding dongs, but it gets me by. I used MyFitnessPal to track everything I eat - there are plenty of good ones, but it really helps to see where your macro counts (Protein, Carbs, Fat).
The one point I’d like to stress above all is not to give up - you will fail - keep trying and eventually you will get there.
Mar21 - BPFSL: 6.5", BPEL: 6.5", NBPEL: 6", MEG: 5.5"
Jan22 - BPFSL: 8" (cold), BPEL: 7.5", NBPEL: 6.875", MEG: 5.5"
One Day - BPFSL: 9.5", BPEL: 9", NBPEL: 8.5", MEG: 6"