Hi Yasmine,
As with all diets, whether or not the weight goes back on afterwards is entirely down to how committed you are to changing your lifestyle. Taking myself as an example - I started LighterLife at over 20 stone at the end of August this year. Now I weight around 16 and a half stone, and I'm aiming to get down to 10 and a half stone by June of next year. If at the end of my weight loss and return to the world of "normal" food, I start eating like I did when I was 20 stone, and keep doing the same amount of exercise that I did then (i.e. none), then yes, ALL of my weight will pile back on, and maybe more.
LighterLife prides itself on counselling its members and changing their attitudes to food. Throughout the classes so far a lot of emphasis has been put on identifying reasons for overeating, and on participating in the "Route to Management" course at the end, which is designed to slowly introduce members to different food groups, and to learn how much food they actually need as part of a healthy diet.
From an entirely personal point of view, I've found this diet to be the best possible option for me, because my problem with diets has always been self-moderation. On LighterLife, I know that I can only have the food I'm given each week, and there's no way for me to tell myself that "just a little bit of X will be OK". Also, the speed of the results (average 3lbs a week) is really good for keeping me motivated.
Your local counsellor should run Information Sessions for people interested in joining the program, where you can watch a DVD and ask any questions you might have. If you're interested in the program and committed to making a change to your life, i.e. you appreciate that it's not just a case of losing weight and then you can happily resume old habits, I'd recommend you contact a counsellor and arrange to go along for the info session.
Best of luck with your weight loss, whatever you decide to try.
Oh and by the way, having just looked at your BMI, it's also worth noting that you would need to do the "lite" version of LighterLife, which involves eating 1 meal per day and 3 LL foodpacks. Again, the info session will tell you about this.