Speaking as someone who has been 20 stone in the past, I would say that my personal experience is that I can not be that weight without having a totally disordered relationship with food that also affects everything else in my life.
I would go to the docs etc and they would say things like 'eat off a smaller plate, swap full fat milk for skimmed milk' etc and I would just look at them like they were completely deranged. Or weightwatchers! Sodding points in a piece of toast! The truth is it takes a huge amount of food to get up to that weight. Huge amount. Fifty calories here or there would just not work for me. A pound of weight is 3,000 or 3,500 calories, depending on who you believe. To lose a stone is a deficit of at least 42 thousand calories.
I say all this because all the well meaning advice I was given by people who'd lost a bit of weight about slimming clubs, haircuts etc was completely useless to me. As someone very wise once said to me, changing diets is just like changing deckchairs on the titanic. Its still going down and you are still going to die.
For me I had to work out how I could live without food being everything to me: comfort, friend, entertainment, lover etc. Food had lost any sense of being physical food to me and become my way of coping with everything in my life.
I think different things ultimately work for different people, we aren't all the same, and we aren't all eating for the same issues/needs, but I think it has to start with the same initial acknowledgement that this isn't about food, it isn't about weight, and changing the food plan etc is not going to do it. Its about getting mentally and emotionally well.